Sometimes it is fun to piece a back to coordinate with the front of a quilt–almost making a two sided quilt. Sometimes it is necessary because of not having enough of the preferred fabric. Sometimes it is prudent because otherwise too much fabric would be needed to match a big print with a long repeat. And sometimes it is a good way to use up the fabric purchased for the quilt. I always try to make it look designed, though, and not just left overs.
PMQG charity quilt kits
There will be a few more as I find the posts