Simple answer, so its out of the way when you're not going to use it, and
it stays on the rifle.
Yes, it makes for less snag in a scabbard. Yes, its so the scope bell clears it. Yes, its so it is out of the way if using a tang sight or receiver peep sight.
That's an issue that does matter to some of us.
Its your gun, and you can just take it off, and toss the sight in a drawer if you want. No problem. Until you want to sell it, or trade it, and the sight has been lost for a dozen years or more...
I once bought a Marlin .44 mag from a coworker. Everything was fine, except when I went to get it from him, it didn't have a rear sight. (this turned out to be a case where he didn't have to take off the sight, he just did it because he felt like it...
)
I took the gun, anyway, with the promise that the rear sight was at his cabin, and he would get it to me. He did. Took him over two months...while the rifle was essentially useless to me. On the plus side I didn't have to buy a new sight...
I think Marlin saved a lot of people from having to buy a new sight with their folding rear, but that was just an additional good effect.