What was Romeo's purpose in going to the masked ball?

Answer 1

Romeo went to the Capulets' party to see Rosaline. But it wasn't a masked ball. Although Romeo and his pals did wear masks, most of the guests did not. Capulet remarks on it, saying "I have seen the day that I have worn a visor", and later he says to his cousin "How long is it now since last yourself and I were in a mask?" The cousin doesn't say, "I'm wearing one now, you dimwit!", he says "By'r lady, thirty years." which means that neither of them are wearing masks at the party--something which would not happen at a masked ball.

Answer 2

It was a custom. Capulet talks about doing the same thing when he was young. "I have seen the day when I have worn a visor, and could tell a whispering tale in a fair lady's ear such as would please." The idea was that, being masked, the young men could boldly flirt with the young women without officially disclosing their identity. Officially declaring their identity might imply that the discussion was serious, and this was only flirting.