I'm thirsty - I desire that my thirst be quenched - so I drink some water. What is my reason for drinking the water? Is my desire itself the reason, or is the reason instead whatever qualities of the object inspire my desire, e.g. the water's thirst-quenching quality? Does it matter?
As a terminological point, it seems that a 'reason' is what answers a call for explanation. Why did/should I drink the water? "Because I wanted to" is not much of an answer. "Because it quenches thirst (and I was thirsty)" seems much better.
On the other hand, we want reasons to be the causes of our actions, and it seems more natural to say that our actions are caused by our beliefs and desires than by external qualities. But these are not really competing causal explanations. Features of the external world explain why we have the particular mental states we do. The world causes our behaviour by means of our beliefs and desires.
So there's a quick reason for favouring the view that reasons are things out in the world, rather than just in the head. I'm still not sure I grasp the significance of the debate, though. It's very much like the objects of perception debate (i.e. whether we perceive the external world or just internal sense-data). It all strikes me as mere wordplay and verbal confusion. The base facts are not in dispute: we have mental states that represent the external world. The only question seems to be how to talk about them sensibly. Or am I missing something?