No-Gi BJJ (also called Vale Tudo) is submission grappling without the use of a traditional Gi, or fighting kimono.
There are less options when fighting an opponent who is not wearing heavy or easily manipulated clothing (i.e. the Gi), thus it is important to train under these conditions often so that one can learn the proper techniques for controlling an opponent in a No-Gi setting.
Technical knowledge of No-Gi Jiu-Jitsu is critical for MMA, as well as self defense on the streets, where you are likely to come up against an opponent not wearing manipulatable clothing.
As a result, roughly half of the classes at CBJJ are taught No-Gi.