in short, the devil refuses to accept that god has made the world as good as it can be, and that's his nature.

So what's the point in the devil? Well, being an omnipotent omniscient being, it would be pretty boring if everyone just agreed with you 100% (as is rational) - so he kills two birds with one stone:

1. He has the devil to debate with

2. He can laugh at the futility of the devil's arguments.

The devil is both his discussion partner and at the same time colossal joke.