Why does anyone drink, period?
lol, more seriously, the way the movie presented it, it looked like SpongeBob and Patrick were preparing to have a fun night out (which for some, means drinking or otherwise becoming inebriated), but it turned instead to a commiseration session because of SpongeBob being upset about not getting the manager position. So then he became inebriated to forget his woes (which is the reason why a lot of people drink). And as Nathan said, for the kids they might understand it as an ice cream headache (though when I was a kid I always saw it as SpongeBob getting wasted LMAO), while the adults understand that this is supposed to look like SpongeBob and Patrick got blackout drunk.