??? Why would your knees touch?

I mean that when in doggy, the leg position changes how the sex itself feels for both partners.

Top legs apart and bottom legs together makes vaginal feel “tighter” but anal feel “looser”.

Top legs together but bottom legs apart switches those.

There are reasons you’d want to do either for either vagina or anal.

No idea what you mean by knees touching at all though since that wouldn’t make any sense at all