What was the roman coliseum use for?

"Theater" - but not the one we know today... you could also call it "circus" - and still not what we think of nowadays...

They used the coliseum to celebrate "games"... it's difficult to say in what years exactly, because not all emperors or governments did support this... "games" means they had fights with gladiators (literally "the one fighting with a sword") or horse races or they had shows glorifying one of their war achievements - this means slaves or gladiators had to play the different nations beating up each other...

Now a bit more exact: There were times when the goal was to see blood... this means gladiators killed each other, or the emperor (and the people) could decide if the winner should kill the loser... and there were even times when they let wild animals (lions, tigers, etc.) kill slaves, i.e. the so-called first christians... because they did not do the same cult as the Romans did to please their gods (Jupiter, Demeter, mars...). But there were other times too, when it was not purpose to kill the opponent (and gladiators were expensive!).

As last comment: Recently researchers and archaeologists found out that the coliseum could literally be put "under water" to glorify roman sea battles :) just imagine!