You don't need a reel...no one does. But as most of us have agreed, if you're looking to be hired in said position, it's a good idea to have one.
As far as directors reels, even if you're not being hired, you might want to have one. It's a good way for people to get a glimpse at your style, what you've done, and your range. It can be something to be proud of. It's also useful as a promo tool for selling previous films you've put out...if someone sees your reel, and likes your style, they may buy some of your films...
And no, not every director is looking to be hired...I know several that make their own films, and simply don't have time to DP or Direct for someone else...some don't even want to.