Quote Originally Posted by KillianHawkeye View Post
I think you're underestimating how bad at simple math some people can be....
It's not just math. It's an organizational challenge. D&D 3.5 has a lot of moving parts to keep track of - even for a simple melee character. In order to conduct combat turns efficiently, you need to know all of the relevant rules for your character, be well-organized, and either have things calculated out in advance so you only have to add one number (the die roll) and maybe one other modifier (e.g. for charging or flanking) on the fly or be very good at tracking all the disparate modifiers in your head. As an alternative, you can have one person who knows all of the relevant rules and mechanics for every player's character do this for everyone, which makes it much smoother when you have players who don't know all the rules very well.

That last piece can also be helpful if you have players who want to be effective (both in and out of combat) but don't have the system mastery to do so and don't have time to acquire said system mastery. It lets people who otherwise wouldn't be able to play to do so, and it also allows people who wouldn't otherwise be able to DM to do so, focusing mostly on story and monter/NPC actions without having to master all of the mechanics.