In CS2 it actually has gameplay changes attached to it. Since it changes where the gun sticks out and where the bullet comes from it has actually gameplay uses too now, hence why the key bind exists.
Earlier games you could change where it appears on your screen with a console command. However only you saw it and it still functioned like a right handed gun would.
CS used to be left-handed by default, and since some people got used to that they kept the option to use either.
In CS2 it actually has gameplay changes attached to it. Since it changes where the gun sticks out and where the bullet comes from it has actually gameplay uses too now, hence why the key bind exists.
Earlier games you could change where it appears on your screen with a console command. However only you saw it and it still functioned like a right handed gun would.