Using belts for squats and deadlifts is a catch-22. To begin with, many novice trainees have no idea as to the true use of this device; day 1 in the gym and they strut with theirs. This approach is utterly WRONG and will make them regret it further down the road.
Belts are useful when handling heavy weight, this is indeed true, but the word heavy is a relative term. It may be considered safe to begin using it when you approach yoiur bodyweight in lifts, but this approach is most unnecessary. The newbie weighing in at a scrawly 120 lbs should have no difficulty WHATSOEVER squatting or deadlifting 120lbs.
A better application of belts would be when your lifts are 2x your body weight. In such a case, a belt can prevent spinal compression( as in squats) and help keep your torso relatively flat (not straight, as it is quite a misleading word) while offering support to your core.
Employing a belt too soon can compromise real strength, and leave you crippled in real life situations where you can't use one. (Think saving someone trapped under a car)