Synonyms:
why the hell
what on earth
why the blazes

Sentences：

Why on earth are you doing this ?

Why on earth are you here?!

Conversation 1
A: What are you still doing here?
B: What do you mean?
A: It’s quarter past six, why on earth aren’t you at work?
B: Oh my god!, I’ve got to run, see you later.

Conversation 2
two girls talking

A: Did you hear about Jane?
B: No, what about her?
A: She’s started going out with Peter.
B: Peter! Is that the really annoying guy, why on earth is she going out with him?
A: Beats me, but she seems happy so I guess that is the most important thing.

(Callan book4, Stage 8 765p)