As mike has mentioned its all to do with the fact that when you are turning a tight corner the inside and outside wheels are going in quite dramatically different sized circles, so need to be pointing in different directions (the inside wheel turns in more)
However that is for a perfect theoretical ackerman angle and there is a lot more to it than that in practice in terms of how the g forces pull the rim around relative to the tyre etc, in some instances its even desirable to deliberately have the opposite effect!