It's just a bit of optimization that allows you to be a little more resilient or versatile. Nothing major.
I made a long post about the Necro feats on the old EU forums (French side.) It's a mess inherited from the merge between the beta Necromancer and Lich (those were two separate classes back then), with more changes from the several balance passes over the years. Not much variety, but at least the class works quite well.
I'm glad to see I'm not the only NF left though, even if I've been focused on pve for quite some time now.