If a certain battle is too hard for you alone, and no friends can help you out, I would suggest you to summon a henchman. It is not a requirement for the game but some players might feel that it is necessary or maybe they play alone and think that the game is too hard. It's personal choice really.
Personally I don't like them. They only last for one a single battle instance and rarely make a big difference. That being said, the higher level Life and Storm can be very useful if you find yourself stuck alone working on a hard dungeon.