One theory is that the "heavier" base "anchors" the font, and stops letters dancing.

I have no idea what the research is like.

This old website has some calm information: (http://www.dyslexic.com/fonts)

Before that, I'd like to see some evidence that they are easier for dyslexic people to read.

The idea is that many letters in the Latin-based alphabet are similar (i/j, v/w, b/d/p, etc.) and dyslexics confuse them. The idea of the fonts is emphasize the differences and de-emphasize the similarities.