I have never played a mage so I don't really know what they do
But I do know that you get extra hit points from them. Some can cast spells. You can use them as 'scouts' b/c unlike summoned creatures, the fog of war is dispelled whereever the familiar goes. And, like 666 said you do lose 1 point of constitution if your familiar dies.
If any of this is incorrect please correct me anybody!
------------------
It is better to keep your mouth shut and appear ignorant,
than open it and remove all doubt!