The internet is older than the world wide web, and that’s just one service that exists.

Having a server directly behind the domain like example.com originally wasn’t common. Back in the days, you might want to make an FTP connection or a pop3 connection, or telnet or gopher, which would typically use like ftp.example.com, mail.example.com, telnet.example.com, gopher.example.com. None of these examples are web servers. You wouldn’t get a webpage back from them.

So when the world wide web became a thing, it was common to set up www.example.com and serve webpages from that subdomain.

The thing that changed in the mid 90s as the internet started to become more popular was that the world wide web quickly took over as the main thing people used when connecting to sites for information or data, so after a while it became common to have the web server respond to requests sent to the bare domain itself. (We’re ignoring ports for the sake of simplicity)

Although that change meant that you could go to example.com for most sites, or www.example.com for most sites, people had already learned that “www dot something dot com” meant a website, and so many people typed www even if it wasn’t necessary. And because there was no harm in having this set up, the tradition continues to this day to set up that subdomain and typically point it to the main web server.

But there’s no reason you have to. You can set things up so that requests to example.com get a different website than www.example.com. But it will confuse your visitors, so most people don’t.

So to answer your question: We stopped needing to say “www” in front of websites URLs by the mid 90s for the most part, but it stuck in the ‘common knowledge’ and is still around today because most people have no clue how it all works. In other words, your question is partly wrong, but the part that’s right has been true for almost all of the public internet’s history. :)

Originally, companies had a much simpler infrastructure and the prefix before a server let you know what sort of server was. So you can imagine something like this:

-mail.microsoft.com

-ftp.microsoft.com

-irc.microsoft.com

-print.microsoft.com

These sorts of services would have been in existence long before the world wide web became commonly used. When it started out, it was logical to put “www” in front of the server name.

Now, DNS is largely used for this purpose. You put “microsoft.com” in your browser and it resolves, and is sent to MSFT. The servers on that end detect what sort of connection it is from the port number used and other methods, then forward your request to the appropriate server automatically. Any variation of it will go to the same place:

-https://microsoft.com/

-https://www.microsoft.com/

-http://microsoft.com/

-microsoft.com

-etc.

Because it used to matter more. As other have said “ftp.domain.extension” was for FTP, mail for mail, and www for websites but that has long been somewhat redundant, as the servers at a domain redirect connections to the right place. mail.domain.extension was originally exclusively for mail servers themselves, but lots of websites long ago made it also valid for landing pages for web interfaces for their mail service if a request comes in via http.

WWW was certainly a very useful concept when people were getting used to the internet. A lot of people had no concept of computers and domains, so to say “www.domain.extension” was clear to people “this is the website for your browser”. And when you say www it’s very clear you’re telling someone it’s a website; that is helpful when say your audience my include elderly people who never got used to computers.

Also the redirection of connections is so simple now, that specifying WWW is generally just not needed. WWW is still the basic landing place for many web sites, but if you don’t include it the servers at the domain will very likely direct you on to the right place, and if they don’t your browser will often try again by appending www anyway and retrying to see if it reaches a valid webpage. So now we just don’t need to tell people “you need to include www”.

Because it’s assumed, like area codes.

www.example.com is a subdomain of example.com. Just like foo.example.com or bar.example.com. Many domains direct their www subdomain to their base domain and vice versa using DNS, but they can resolve to different IP addresses.

www was just a convention for notating that the server was for a website on the “world wide web”. As opposed to mail.example.com being for an email server or ftp.example.com being for a File Transfer Protocol server.

Because of the implication

Some websites still have it, for example youtube.com and openstreetmap.org redirect to those URLs with www. in front of them at least for me.

Other comments already correctly explained that the original idea was that the third level domain would indicate what kind of server it is: www.example.com as opposed to irc.example.com, ftp.example.com, smtp.example.com, whatever. That convention existed before the WWW was invented; when it was, it was just yet another kind of service that existed on the Internet.

The WWW has by now developed into pretty much the default service people use the Internet for, so it’s usually no longer necessary to add www., although it’s still technically possible for www.example.com to point elsewhere than example.com, or even for one of them to work and the other not. Whether the version with www. redirects to the one without or the other way round, is something most people don’t even notice…

As for why we don’t say it, the reason is approximately the same: the WWW is now generally understood to be the default service of the Internet (many people don’t even know they are different things), so when you mention a domain name nowadays, it’s assumed by default that you are talking about what’s served on that domain on the web, not something else.

When giving a web address, I’ve heard someone say the entire url with the www and https, if was a little crazy to hear

Anything in front of the . is a subdomain: “subdomain.domain.com”.

When webpages new, it was convention to use the subdomain for web traffic. Technically you can still set up your site and make www.mydomain.com different from domain.com, but pretty much everyone just makes one their primary site and then redirects the other one. So you set up www.mydomain.com to redirect to mydomain.com and just use that one for your main site, or vice versa.

In Japan, many still do and, if you forget, you often don’t get automatically redirected but rather get an error page.

Mainly because it’s lost it’s novelity. all browsers automatically search for it so it’s not something people need.

We need mmm.example.com

Do write 3 less letters

Because saying “www” is a mouthful and serves no purpose to express vocally these days.

Old people still do. And they also mention capital letters in their email as if capitalization does anything.