From Mark Andreessen:
Whenever you see a successful startup, you see one that has reached product/market fit — and usually along the way screwed up all kinds of other things, from channel model to pipeline development strategy to marketing plan to press relations to compensation policies to the CEO sleeping with the venture capitalist. And the startup is still successful.
Conversely, you see a surprising number of really well-run startups that have all aspects of operations completely buttoned down, HR policies in place, great sales model, thoroughly thought-through marketing plan, great interview processes, outstanding catered food, 30″ monitors for all the programmers, top tier VCs on the board — heading straight off a cliff due to not ever finding product/market fit.
Ironically, once a startup is successful, and you ask the founders what made it successful, they will usually cite all kinds of things that had nothing to do with it. People are terrible at understanding causation. But in almost every case, the cause was actually product/market fit.
Because, really, what else could it possibly be?