To ensure that the jpeg doesn't accidentally put an ever-so-slight discoloration on the actual quilt tile. I know it won't just as a png, but when the jpg is generated to trim the size of the quilt down somewhat, the checkerboard will sometimes leave slight artifacts on the border. Another reason to always draw outside the edges and never send your tile with pure white (or pure anything, for that matter) replacing the checkerboard.

That's interesting to know, thanks for the tip Gecky!
I'll have to start smudging into my checkers now...

Stupid Typo, I meant Gecko, not Gecky.