I’ve heard a few sources say that exercise helps with maintaining weight (keeping weight off) and has other health benefits, but has been found to be at least of fairly minor effectiveness for weight loss compared to other factors like diet. I believe part of the explanation is that over the long term the body will otherwise adjust to the excercise expendature in various ways (reduce energy/metabolism so less of a workout is possible, reduce energy/metabolism outside of the workout, activate hunger and appetite to greater degrees, etc).
Dr. Fung has a multi-part series on the issue on his blog (and I believe touches on it in his lecture series on obesity):
The Myth about Exercise – Exercise Part I