- Look/feel changes: These simply change the appearance of the GUI. Sometimes they add more information, like overlaying a % over the enemies health bar, or highlighting party members who are taking damage.
- Bars: Many of the early addons were to add more buttons, until Blizzard implemented this themselves.
- Raid: lots of addons help with raiding, like showing the health of everyone in the raid, showing the main tank's target, etc. Also, debuff cleansing with one button press (scans the raid and casts an appropriate cure spell), though this is being disabled by Blizzard in the coming expansion.
- Additional GUI: some mods don't just modify the existing GUI they add more. A bar across the top listing money, regen rate, your (x,y) location on the map, your XP/hour, your fps and ping, the amount of ammo you have left, etc. For classes with reagent needs it can track those.
As you can see most add-ons revolve around giving the user more information. The closest thing to botting there is the auto-curing, which is on its way out.