Why do they make different sites for mobile and desktop versions of web applications?

Question

css

Good afternoon.
Interested in such a thing, I see a lot of sites that have different addresses for mobile and desktop versions. Often, their functionality is similar, only styles differ, and the address formats are as follows: site.com (for desktop), m.site.com (for mobile). You can also make responsive styles media queries.
Yes, there is such that the functionality in the versions is different, but there is a similar one. And how, in this case, implement a new feature on two versions? Saw separately the code for the first and second?
I would be grateful for the explanations :)

If I messed up with the tags - sorry, suggest editing.

Answer 1

page loading speed (desktops can be quite heavy), PWA, AMP, Turbo pages, etc.