A familiar will follow you around, but you can also control it independently, see through it's eyes, command it to attack.
Some of them can open doors or pick things up.
Some of them can see hidden or camo.
Some of them can fly above Thera.
Personally, for a good conjie I like the cherub. He tanks pretty well with the two familiar edges and he helps with your angel/archon conjures.
However, if you're going to explore you may want one of the ones that can detect, so it's really about what you want coupled with the fact
that you have no choice in what familiar you get.

Your servitors will follow you around. You can roughly gauge how happy they are by howw quickly the follow you.
If you initiate combat, they'll jump in. An angel may rescue you, an archon will heal you. Elementals will hit your foe and you have rthe option
of having them do area attacks as well. Each elemental has various other functions that you can order it to do.