Are parking meters just revenue raisers?

Why should people get fined for parking their car for longer than the allocated time, even when there are plenty of free parking spaces? Isn’t the point of parking meters that when you are parked there for over the allowed time, you’re taking away what could be someone else’s parking space… but if there are plenty of spaces anyway, does it really matter if you are parked there for too long?

(By the way, I’m talking about parking on the side of the road where there are parking meters – not, say, shopping centre car parks which are obviously just revenue raising)