No, it's just for the weed suppression. It sprouts quickly and has decent sized leaves right away, shading out the weeds that sprout underneath it.
I plan to leave it until frost kills it. If it flowers, so much the better! Honey bees like it.
I'm thinking I might do a patch every year, and rotate where that is. One year, I did three courses of buckwheat in one part of my garden. When it flowered, I took it down with a string trimmer and left it sit for a couple weeks. (We didn't have bees back then.) I did two more plantings. There were VERY few weeds in that part of the garden the next spring.
Buckwheat does not fix nitrogen, but it does add plant matter to the soil.