http://www.submissioncontrol.com/mai...-or-not-to-gi/
This is an part of article one of my blackbelt instructors wrote
"The technical aspect of gi training has to do with the fabric of the uniform itself. The gi is made of heavy-weight cotton, which absorbs sweat and adds a tremendous amount of friction when applying submission holds, making escaping extremely difficult. The gi also provides countless handles to secure limbs, assuring your opponent doesn’t just slip away.
While strength and speed are not the most important attributes in BJJ, they can certainly help. The lack of fabric in no-gi training makes absorbing sweat almost nonexistent, leading to a slippery match. This is one of the biggest differences in gi and no-gi training: submissions need to be spot-on to end a match; an opponent in a no-gi contest is more likely to slide out of a submission attempt and turn the match around."
Basically he's saying(and I agree with this) wearing the Gi can help make your defenses tighter, while No-Gi will help make your offense tighter.
Most of the people at my school swear by the Gi, I on the other hand could do without it and prefer training without it. To each his own I suppose.