I believe, as do many others, that the roll cast is the second most important cast to learn. The roll cast can be used in many situations. Off the top of my head (1) when there is no room behind you for a backcast (2) when the wind is coming from your back and interferes with your backcast (3) the roll cast can be very useful in getting your fly line in the air quicker to begin another cast because you do not have to strip the line in as much as would otherwise be the case.

Making the roll cast Orvis-Pete Kutzer

The roll cast: by Fly Fishing for Beginners - (article and video)

The roll cast - Dusty Wissmath (YouTube)

The roll cast -John Wilson, The New Fly Fisher (YouTube)

Roll Casting by Fly Fishing International (article)