Donor Kits are a reward in which a user can occasionally redeem for a bundle of items/bonuses. They have a cool-down for balancing purposes, and typically will only give the user a small boost.
/kits - Opens the Kit GUI. You can issue all available Kits from this GUI by clicking the associated icon.
/kit <name> - Issues the kit that is specified in the command. If there is an overflow of items, they will drop on the ground.
/kitpreview <name> - Opens a GUI which will show the physical contents of a Kit. If items are issued through command, they will not appear here.
Simply put, there was no good way to balance the regular Kit system. A consensus was reached where balancing this game element was going to end up with Kits being too overpowered, or too under-powered. Rather than disappointing people with a bad level reward, or breaking the game with a minor element, it was felt that the best course of action was to just move away from Kits as a whole.