Racing F1: What made Alain Prost a "tactical genius"?

I’ve watched the guy on a couple of old tapes. I will admit that one impressive feature he does have is that he manages to sort of win without winning (or going fastest - for lack of a better word). He kind of comes up on the drivers stealthily - and though I found him by far the most fascinating to watch, I just don’t get why people rate him such a “tactically astute mind” (at least according to Clive James).

Bear in mind I was watching races from the '84 and '85 season. Maybe his mental sharpness wasn’t quite as prolific as it (maybe) was in later years. I only started watching Formula One from 2002 so I’m not quite so aware of it’s history - but what exactly is there so much to be “tactical” about? I mean I know car set-up is important; but most of that is done by the teams right? How much input can a driver have? And even if it is quite substantial, I don’t know why you would give Prost so much credit for it.