You can improve shield skills only
The protection tree makes the Paladin a better tank, giving better shield skills, better aggro gaining and such. You are able to build this tree up to Blessing of Kings, which is arguably the most useful buff you have. Healers and casters should not have this buff on, as Requiem gold is more important, which depends on where you use it. When getting the Blessing of Kings, you can improve shield skills only if you want to off-tank a lot or get 10% more amour, who applies when you use 2h too.