Conversation so far:
What is Tara's primary role?
Throughout season 4, Tara acts as a partner and guide in Willow's witchcraft, teaching Willow spells and performing magic together.
Do they form a relationship?
Tara's primary role throughout the series is that of Willow's partner.
Does Tara get Willow into magic?
When Tara and Willow meet, their proficiency at magic is about the same, but Tara's knowledge of the craft far exceeds Willow's.
What keeps them grounded on use of magic?
CANNOTANSWER.
Does Tara have any doubts about magic?
She feels somewhat useless until the fifth season episode "Family" when the entire Scooby Gang (Spike included) makes it clear that she is unquestionably a part of them.

Question: What makes her so useful to the gang?
Self-contained rewrite: What makes Tara so useful to the gang?