Having a nuclear test ban treaty set in place would be beneficial to the risks of a potential nuclear fallout. Banning the testing of nuclear devices and weapons would reduce the risks associated with carry on on such a dangerous task. Nuclear material is extremely dangerous and it is unknown how much damage it could cause in a fallout.
If countries are not able to test their nuclear weapons it will lessen the change of a nuclear fallout. There are many things that could go wrong with a nuclear test. The weapons could go off early or end up in the wrong place. Plus, it is bad for the environment whenever a nuke is set of.