Normally when you work a cast-on it is complete and you won't return to it. A provisional cast-on is a little different, you are able to undo it and knit from the cast-on after you are finished.
There are several different situations where this is useful:
The two types of Provisional Cast-On I use most often are the Crochet Method (plus how to undo) and the Invisible Method.
When you work a provisional cast-on you will be working in the other direction. For this reason you will be 'between' the original stitches so you will need to pick up the edge loop to ensure you have the same number of stitches.
If you are working a stitch pattern ensure that it works from the other direction. It is best when you start work from a provisional cast-on to use a different stitch pattern.
I frequently use a provisional cast-on to join seamlessly. I've used it when working projects from side-to-side:
