Corrected entry: Why did Phoebe have to write a spell to vanquish the warlock in this episode? When they have vanquished warlocks before they used the power of three spell, "the power of three will set us free". (00:16:55)
Correction: There was actually something in their contract about not having to say the Power of Three spell too many times per season, or something to that effect. I can't remember the exact way it was written, but neither the writers nor the actresses wanted to say that spell all the time. So they only saved it for dire situations, instead of just using it to vanquish everything (because technically, they could've vanquished every demon, warlock, etc. with that spell).
Correction: Spells work best when written to deal with a specific demon or type of demon. There are many demons who they write spells for, and others they simply use what worked for their ancestors who then wrote the spell in the Book of Shadows.