Cari-in-Oly

I think it might be for keeping the edge of the top away from the leaders so that the quilting can be done right up to or off the edge of the quilt without running into the leaders.Of course this would be done if the top were loaded sideways on the frame.
I once had a minky backing that was barely bigger than my quilt top so I heavily thread basted the two layers together, then sewed a strip of muslin all the way around the backing and loaded it on my frame like that to quilt it. When I was done I trimmed up the backing, cutting off the muslin strips. Worked great.

Cari

Edit - I think Quiltingshorttimer has a better explanation than my thought.