It depends on your correspondences, what sort of spell you wish to perform, how you wish to perform it, whether you have the time and patience to wait for the phase of the moon to shift, etc. In my experience it helps with the subconscious self to get further into the working, but if you are performing it properly you don't necessarily need the moon in order to get it to work.
In ancient Egypt, the moon corresponded to a lot of beings, and had a lot of meanings. The most common thing I've seen in regards to it is that it represents the damaged Eye of Heru-sa-Aset, and as such during times when it was full people rejoiced, when it was dark they mourned for Heru-sa-Aset's lost eye, when it waned they rejoiced as it gained strength, and while it waxed they feared as his power weakened. I've read that during New Moon nights people would lock their doors and sit in fear that isfet would have finally overpowered ma'at and that the forces of evil and chaos would reign. When the light started to return to it people would celebrate that Hetheru (Hathor) had succeeded in returning his sight and that the forces of isfet would be held back once again. I may be a bit off as it's been a bit since I've researched into this and I've been meaning to get more involved with the calendar systems.
It's really dependent on where the tradition/belief system one is taking it from and what it means.