It seems a long standing tradition perhaps at the outset of a new Gran Turismo, to complain about not having enough credits. At least it seems that way since I've been playing (GT4), and my husband says even before (think he started with 2 or 3).
And then eventually, you wind up with more than you need.
Everyone's always anxious for more sooner in the beginning of anything I think.
Microtransactions (aka buying credits) don't seem to be half as prominent in GT6 as in some other video games.
And frankly I think they're there just for people who have more money than patience at the beginning starting out!
In other words, they're especially for the people who complain about not having enough credits to start out with!
Also, what Famine said occurred to me as I started winning cars.
I wasn't really worried about having to buy cars to complete stuff, as I have the gamestop anniversary cars, and I've already used a couple of those to complete stuff. But I did notice that the 1st 2 prize cars I got, would be useful for other stuff I saw upcoming. Even so, looks like without the prize cars you can afford most of what you need as you go along just by getting credits from races. Just that you wouldn't be rolling in it, you'd be cutting it pretty close. Which is what I think some people enjoy - working with a budget is part of the game for some.
Not to say I'm pleased with the prize car list in comparison just generally speaking. Seems rather dull - in terms of my car preferences.
But yeah, probably more useful for completing the career events.