Why does batman wear pants - over - clothes?

[superboy]

 ???I thought about this the other day and can't think why batman should wear underpants over his costume.Superman ( or at least silver age superman wore it because it was kryptonian fashion, but why should bats ?I know the idea of a guy dressing up as a bat is a bit crazy anyway, but that has it's crazy reasons.

[omarromerro]

Actually he has setup his own trend, which will last forever and ever. It's his trademark.

[superboy]

good point. Although superman should sue bats for copyright issues.