undersized throats will deform the bullet before it enters the forcing cone and then engages the rifling causing poor accuracy, this problem is most common in 45 cal Rugers but most guns will benefit to one extent or another from reaming as it makes all the throats consistent

I won't be wronged. I won't be insulted. I won't be laid a-hand on. I don't do these things to other people, and I require the same from them. John Wayne-The Shootist