It's a complicated legacy of many cooks in the kitchen.
Add/Remove was orginally for non-networked users to mage the dvd groups.
It can now turn on or off a repo, also, permently until the repo os turned back on.
yum is the tool to use and has been improved to the point of being half calculator with complex cammands and many checks.
packagekit is in the mix but not ready to take over add/remove.
So, if you want to be a cook go to the documentation and learn about add/memove and before you learn it it will be just another gui for packagekit, which is a gui for yum.
If you want to eat delisous rpm's, use yum.
yum is the man, the moon, the sun and the stars.
Add/remove is a potato peeler.