The belt is used to create extra intra-abdominal pressure - basically, taking a big billy breath and forcing your stomach out against the belt helps you keep your midsection tight.
so no, the belt isn’t “mandatory” but if being tighter in the midsection can help you get a few more pounds, sure, why not?
If you’re a complete beginner, I would say that you should learn HOW to effectively wear and use a belt, but train without it as much as you can. You’re never going to run into the problem of “my midsection is too strong” but you can certainly encounter problems if your midsection isn’t strong enough.
In general, I do all my sets at 85% or below beltless. I pretty much only put the belt on when it’s time to go heavy and push myself.
Paul Carter from Lift-Run-Bang wrote an article on this earlier this week:
And I tend to agree with him. If your abs and lower back are comparatively weak, and you’re totally dependent on a belt, that’s not a good sign.