I think it's just one of the possible ways of saying the same thing. For example, in English:

Почему [нет]? => Why not?
Почему бы и нет? => Why wouldn't you/me/it?

And whilst English speakers would typically prefer the former, Russian speakers prefer the latter. I suspect that is because "нет" and "not" are used differently. In English "not" would serve an integral part of the negative sentence.

- I do not want to go there.
- Why not?

- Я не хочу туда идти.
- Почему?