One of the things I like about it is it's just a 10-12 week plant. That means you can plant it into early June and it will just be finishing up about the time you want to plant fall plantings.
It also makes a great "cover crop," in that it can be mixed in lightly (10-20 pounds per acre) with other mixes. Because it comes up so quickly, deer spend their time browsing the buckwheat instead of the plants you are really interested in growing. Nothing knocks down beans and peas faster than early browse pressure (just after germination). Using buckwheat to take the browse pressure of of these more desirable plants gives them a chance to grow to the stage they can handle browse pressure better.
I use buckwheat even in my fall plantings. It matures out just about the time the first frost comes along (which would have killed it anyway).