M.D.
My son takes fluoride drops because our towns water supply does not contain fluoride. He loves his drops because they are yummy peach-flavoured. He calls it "med-sin", and is excited to brush his teeth because he knows he gets the it afterwards.
If your childs doctor gave her a prescription for fluoride, its because it is assumed that your well water does not contain it. You can have the town come check your well to see if it contains naturally occurring fluoride, in which case your daughter shouldn't need to take the drops. Once your child is old enough to use regular toothpaste, which usually contains fluoride, she will no longer need to take the supplemental drops.
The purpose of fluoride is to prevent cavities and strengthen the teeth. You asked if it is necessary, and I would say yes.