They do it for a high - they're usually not allowed to drink after a game by their clubs, they obviously can't risk illegal drugs that could see them banned, so they do this for kicks... Members of the NZ World Cup team were doing it and got caught after the semi final win when one of them turned up to training the next day still off his face.
The real issue with this is that it's highly dangerous, and they're often taking painkillers (which may or may not be taken for legitimate reasons) and then the sleepers and energy drinks for the kicks... Dylan Walker and Aaron Gray both ended up in serious conditions at the end of last season doing exactly this - they were luckily found by a friend with one of them choking on his own vomit and the other already in a coma... if they'd not been found when they were, the likelihood is they'd have died.
And it all stems from a much bigger issue around prescription painkillers in general. It's well known that many (if not most) players now rely on painkillers to recover, train and play, they're handed out like sweets by club doctors..... The side effects of the painkillers are lack of sleep, so then they need sleepers to counteract that, then the more they rely on painkillers, the more they need to take, the more painkillers they're on, the more sleepers they need... It's a viscous circle and will ultimately lead to a player dying if it's not controlled.
Malcolm Alker talks about the issue in detail in his autobiography. The authorities are currently turning a blind eye and burying their heads in the sand on this one - I fear it it will cost someone their life before the issue is tackled properly...