The idea is to have a buffer. You don't want to sell every car on your lot and have to wait for a daily shipment. Sales are not steady, there are busy times and less busy times. The factory runs on an average of how many sales are expected.
People also like having a selection.
If the average vehicle takes 60 to 75 days from the time it was made till it is sold to the consumer, that is OK. It's when the cars are not selling and they start to be on dealer lots for months and months that it is a problem.