The dashes fix an ambiguity problem. 12031102? Nov 2, 1203 or Dec 3, 1102 or March 12, 1102. It's very, very rare to find YYYY-DD-MM. So if you're interpreting human generated dates, the dashes pretty much force YYYY-MM-DD. And YYYY-M-D is easily interpreted as a bonus.
As others have noted, file names really, really want to be in numeric, YYYY-MM-DD order. Take note when you supply your users the file name when they download from you!