Actually some fighters wear them weeks before the event if they have alot of weight to lose and they know they couldn't keep it down. For example, the three weeks before his 1st bout vs.Pacquiao, Erik Morales did all of his sparring and ring work in 3 layers of clothing - at altitude!
Generally speaking, this is very dangerous and unhealthy if it's not done by athleates who's bodies are acustomed to such ordeals, and with medical supervision.
Guys who are naturally beefy or muscular can really weaken themselves by doing this. Conversley, in the case of Morales, he's a skinny man and he needs to perform skinny no matter how gruelling the weight loss. When he fought just 5 pounds heavier last year, he looked ridiculously sluggish.