Why does atan2 take 2 arguments?

[Jorg Ziefle]

I wondered since quite a time why atan2 takes 2 arguments.

This doesn't make much sense to me since you can always calculate you
own "slope" and pass it to atan2.

If there is some issue in this that I oversaw, could anybody please
explain it. If not, I'd like to suggest to skip the 2nd argument.
-- 
Jörg