In the case of women, especially after having a baby, the abdominals are weak. They took nine months to get out of shape and they'll take a few months to come around. If you just let them go, you'll be prone to back pain. For men, the beer belly that you're working on reducing sets you up to having back pain because it pulls the spine into a sway and offers no support for your back. Until your abdominal muscles become strong enough to support your back, use a belt or support.
Belts increase the abdominal support by providing support from the front of the spine.
If you have to lift for a living, or if your job necessitates loading, bending, carrying, standing reaching all day, then use a belt. I'm sure you've seen workers wearing them. Over the years we've recommended belts with great success. Recognize that using a belt isn't intended to replace your abdominal strengthening program, but rather as a support until your abdominal musculature gets strong enough to carry the load. Even when your abs do get strong, it's o.k. to use a belt if you have a job that requires prolonged heavy lifting.