Most people don't know exactly what a belt is supposed to do... so here ya go...
The purpose of a weight belt is to increase intra-abdominal pressure. When lifting without a belt, the spinal column is supported by the fluid contents and musculature of the abdominal cavity. When lifting near maximal weight, the diaphragmatic muscles contract and put pressure on the abdominal cavity, stabilizing the spine. Using a belt helps increase this pressure, while reducing the load on the musculature of the lower back. Sounds good, right?
Well, if you use a belt when it's not needed (lat pull downs for example), it takes the place of your core muscles. Since you're not using your core stabilizers, they can atrophy, leading to injury. And since they have to be tightened to be effective, they can raise blood pressure.
So, if you're going for max lifts that only require 1 rep, a belt may help. But if you're going for reps at submaximal weight, go without the belt.