IN
Can someone explain to a newbie the point of making weight if you’re allowed to just balloon up the next day?
The point is that fighters of similar physical stature/build will generally compete in the same weight class.
People who can't make weight are fined, risk having the fight cancelled, and even blackballed for not making weight. Thus, fighters are less likely to fight in a division where they will likely fail to make weight.
Rehydrating is accepted as being part of the sport, so long as you make the weight first. There are also people who believe that they should bring back same day weigh ins instead of doing them the day before the fight.
The IBF also enforces a rehydration limit for their belt.