TLDR - because its needed.
First of all, I believe car use should be discouraged so bit biased here. Just getting it out in the open.
The idea was that public transportation was (and still is) a massive issue in Auckland. Split by two harbours creating choke points, massive sprawl, complete lack of investment. Can't just keep adding more cars, more lanes, more delays. Why not take a bit from the individual problem causer and use it to greater a better long term solution?
I've been stuck in traffic for ages, then look beside me and see a bus stop that didn't get finished (Rosedale road for anyone around) because it got bumped for budget. I use a car for work where public transport isn't practical (10 sites a day with full kit) but when I do site visits for new clients the first thing I do is check for a bus.
Oh, and given up on trying to get a car parking in cbd - $14 for 30 minutes parking after 40 minute drive? Na, I'm good, id rather take a 30 minute bus trip and meet the wife for lunch.