I do not use it and would advise any client against it.
I teach my clients to protect their spine without further need for external support and only use the amount of resistance that can be handled without compromising that goal.
Having said that, if people are in competition where they go to the limits of their strength, then there is a place for those belts. But even there, I would hope that the athletes would do all they can to work the stabilizing muscles to the greatest extent possible.
If youre working with a client in a rehabilitative or wellness regime, no.
If their aiming to lose weight or get better athletic abilities, you are challenging them to the best of their capabilities.
Yes, get a weight belt for someone who maxes out on squats, deadlifts or upright rows.
Dont get a weight belt for you 8am 40yr female client.
Everything has a place in the gym, you simply need to take into account who youre working with and what their goals are.