for what reason or purpose

why did he do it?

(with reference to a reason) on account of which; for which

the reason why flu jabs need repeating every year is that the virus changes

expressing surprise or indignation

why, that's absurd!

used to add emphasis to a response

‘You think so?’ ‘Why, yes.’

a reason or explanation

the whys and wherefores of these procedures need to be explained to students