Well, the issue is that when you are being rewarded for the work you are doing, the motivation for why you are doing it changes: and the higher is the reward, the less you stay interested in doing things (with very rare exceptions). That was noticed during some researches [wanted to reference some, though can't find links quickly], and it kind of makes a definition of "work" to work differently than "have a passion and you won't work a day".

That is a reason why we need this distinction: not all people need to be incentivised by money for literally everythign they do. Sometimes people need to do something just because they want to, over what they need to to get going with their lives.