First, you've got it backwards. The upper gel buffer is 6.8, and the lower is 8.8.

Here's the reason:

The well is about 1cm in thickness, so the proteins in the bottom of the sample have about a 1cm head start compared to the proteins in the top of the sample loaded into the well. Without an upper gel, maybe your bands would all be 1cm thick. The upper gel is designed to concentrate all proteins into a narrow band so that they all begin migrating through the lower gel at the same time. At pH6.8 glycine has a neutral charge, so it migrates very slowly and all proteins get concentrated in a narrow band between glycine and Cl- (from the Tris-HCl) until they reach the lower gel (pH 8.8), where glycine acquires a net negative charge and greatly accelerates, as do all of the proteins. Since the lower gel has a higher concentration of acrylamide, larger proteins will travel more slowly than smaller ones.

Here's a nice lay description:

http://bitesizebio.com/articles/how-sds-page-works/

Thank you so much :)