Question: What's the difference between:

> 
> I want it
> 
> 
> 

> 
> I want to have it
> 
> 
> 

And what is the reason?

Answer: 
Cheers Ringo for great examples and sound reasoning. I wasn't sure but you have me sold. Rereading this I get a feeling that "to have" has a slightly different feel to it, using the Ferrari example (as an example), "I want to have" says something about continued possession of the Ferrari but I'm just sleep deprived and also really bad at both speaking and comprehending the English language and you obviously are not.
There is a slight difference between the two. So little that both can be used in place of each other without changing the actual meaning of the sentence.
'want to have it" is a stronger urge to get something than 'want it'.