The UVB light is supposed aid in the development of trichomes, THC, and overall potency. I've heard that UVB irritates the plant and causes it to produce more trichomes in an effort to protect itself, but I'm not sure if its true.
I used some in my first grow and did notice an increase in trichomes after I added them. I didn't use them in my grows since and have not seen a difference in potency from my first grow.
This is the thread that i first heard of using UVB bulbs.
https://www.rollitup.org/advanced-marijuana-cultivation/36368-trichomes-thc-uvb-light.html
*edit: Marijuana man made a video about UVB, check it out
http://www.youtube.com/watch?v=IPcpt3Be28o