If you mean laser sights, they’re faster to aim with than scopes, so you come to a low level of dispersion faster, then use the sight. Overall it doesn’t make a lot of difference but having one is better than not for close shots.
Technically they still reduce it’s dispersion, but it only has a dispersion value of 10 (which is then divided by 16) so it has less than 1 dispersion ingame, which isn’t shown. The grips will get rid of that invisible bit of dispersion AND will offset any other mods that increase dispersion.
I think they’re for homemade weapons that haven’t got real iron sights. I don’t know of any guns like that, but that might just mean it’s deprecated and doesn’t need to exist anymore.
There’s no realistic reason why you COULDN’T put a brass catcher on a crossbow, but it obviously isn’t going to do anything.
All gunmods say what they do, if you stick something on your gun that isn’t going to have an effect that just means you didn’t read the description.