There are many ways to do a provisional cast on. Right now I really like the turkish cast on, which I have been using on socks. There is also a version involving a crochet hook that I really like.
Provisional cast ons leave a row of live stitches where you would usually have a bound edge. This lets you knit in the other direction. An example where this would be useful would be a scarf where you wanted to start in the middle and work out toward the ends for a symmetrical design.
I wouldn't worry about learning how to do a provisional cast on until you need to use it. There are so many different methods and the one you choose may very well depend on the particular project you're working on. HTH