Why is glycerol used in SDS-PAGE?

Glycerol is added to the loading buffer to increase density of the sample, so it does not diffuse out of the gel. Glycerol is denser than water. When added to the sample, it acts as a weight on the sample, causing the sample to drop to the bottom of the loading wells. This prevents it from diffusing out of the well and into the running buffer. 

In case the samples start floating up in the well, adding more glycerol to the sample can help keep it down without affecting the proteins themselves.