Conversation so far:
What is Tara Maclay's primary role on the show Buffy the Vampire Slayer?
Tara Maclay's primary role throughout the series is that of Willow Rosenberg's partner.
Do Tara Maclay and Willow Rosenberg form a relationship on Buffy the Vampire Slayer?
Tara Maclay falls in love with Willow Rosenberg.
Does Tara Maclay get Willow Rosenberg into magic on Buffy the Vampire Slayer?
Tara Maclay acts as a partner and guide in Willow Rosenberg's witchcraft.
What keeps Tara Maclay and Willow Rosenberg grounded on the use of magic on Buffy the Vampire Slayer?
Tara Maclay's name resembles the Latin terra, meaning earth. She is solidly grounded, with Willow Rosenberg attached to her.
Does Tara Maclay have any doubts about magic on Buffy the Vampire Slayer?
Tara Maclay feels somewhat useless until the fifth season episode Family“.

Question: What makes her so useful to the gang?
Self-contained rewrite: What makes Tara Maclay so useful to the gang in Buffy the Vampire Slayer?