Yes, it's all about low light and sharpness. I didn't fully appreciate the speed of a 1.4 or 1.8 lens until I was tasked with photographing bands on stage w/o a flash. And fast lenses earn their wage when it comes to situations like this.
As Dave mentioned, "faster" lenses (i.e. lower 'f' numbers) mean better light gathering with less light required. Also remember that most (well-built) primes have less glass than is usually involved in a zoom so they are capable of better overall image quality (sharper, better color, etc).
For the most part, primes are sharper across the entire field than zooms. The lower f stops gets you faster shutter speeds (hence better in low light), with a corresponding loss of depth of field. You get a lot for your money, but you loose the convience of zooms.