“Free will” is probably not the best guide to the thing we want to talk about. “Will” connotes assertion, imposition, the fiat that brings something forth; “free” means absence of constraint or control. Put the two together, and it sounds like what we’re fighting about the unconstrained assertion or a chaotic fiat. This is why people have been objecting since Chesterton’s time that “free will” would be a sort of mania or madness: Mr. R.B. Suthers said that free will was lunacy, because it meant causeless actions, and the actions of a lunatic would be causeless (Orthodoxy, c. 2 “The Maniac”).
The better term is simply “choice”. Not “free choice” – which is pleonasm – just choice, i.e. the rational decision for one of many real alternatives, where “real” means “not just conceivable or logically possible”. Determinism merely denies the reality of alternatives – one can conceive of acting otherwise but it was never a real possibility.
So taken, the fight between determinism and its opposite seems very esoteric: it involves trying to come to terms with the ontological status of things that do not happen. But, as Aristotle points out, the debate does have one crucial outcome: it seems to determine the value of deliberation. Deliberative processes rest on the supposition that something is really at stake, and so we need to collect evidence, sift through it, weigh arguments, and try to do our best to pick between which really live option is the best. Determinism has a very difficult time accounting for why we should care about this.