Question: What's the difference between:

> 
> I want it
> 
> 
> 

> 
> I want to have it
> 
> 
> 

And what is the reason?

Answer: 
There isn't much difference, as far as I can tell. There might be more immediacy and emphasis to ***want*** than ***want to have***. For example:

> 
> He wants sex.
> 
> 
> He wants to have sex.
> 
> 
> 

People might disagree, but I think ***he wants sex*** could connote a stronger or more spontaneous urge than ***he wants to have sex***, which connotes a more deliberate or more formalized or less immediate action.

EDIT: Adding some more examples just so people can draw conclusions for themselves:

> 
> I want the kidney transplant.
> 
> 
> I want to have the kidney transplant.
> 
> 
> The CEO wanted the Ferrari.
> 
> 
> The CEO wanted to have the Ferrari.
> 
> 
> The dog wants a kiss.
> 
> 
> The dog wants to have a kiss.
> 
> 
> We wanted a World Series championship.
> 
> 
> We wanted to have a World Series championship.
> 
> 
> 

Looking at these examples, I still believe ***wants to have*** implies a more deliberate or more formalized action. The desire in ***wants*** is perhaps more ephemeral or spontaneous.

For example, it's a tiny bit awkward to say ***The dog wants to have a kiss*** because it implies that the dog is thinking about it and has made a conscious decision that he would like you to kiss him. We know dogs don't reason in this way, so it's a little humorous to say a dog wants to have a kiss. Instead, we would say, "The dog wants a kiss." It's a simple, spontaneous desire.
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'.