Here's a simple model of human behaviour: we have beliefs and desires, and we act so as to fulfill our most and strongest desires given our beliefs. (I think Alonzo Fyfe holds something along these lines.) On this view, call it 'BDI', our intentions (the psychological precursors to action) are wholly determined by our prior states of belief and desire. But if that were so, there would be no need for practical reasoning or deliberation. The mechanism for converting beliefs and desires into intentions might as well be sub-personal, like an automatic reflex. Instead, the phenomena suggest that there is a further element to agency, not reducible to beliefs and desires, which we may call the 'will'. (N.B. It may be reducible to some other aspect of neurological function; I do not claim it is non-physical.)
Start with theoretical (epistemic) reasoning. Does anyone think that our conclusions (new beliefs) are wholly determined by our prior states of belief? They're a huge factor, no doubt; what we find plausible will depend on what we already accept as true. But what new conclusions I draw - if any - will also depend on how much attention I pay to various reasons, how carefully I consider the issue, and so (probably) what I had for breakfast, among other things. There is room here to identify a causally-embedded 'will', which weighs and assesses the various arguments and reasons that fall under the spotlight of its attention. The way it functions is presumably determined by the totality of my brain states, but not - I think - my beliefs alone.
Similarly with practical reasoning. Desires may now enter the picture, but they seem to make little essential difference. Much still depends, for example, on which desires (or other practical reasons) we attend to, and how we will weigh and assess them -- it is not predetermined by these states alone. There is a distinct psychological faculty in play here.
(This also explains how we can come to bad conclusions, failing to do or believe what we have most internal reason for. Sometimes we just overlook things.)