Since you don't have a clutch, you can use the automatic transmission. That's why it's there. Like others have said, the automatic transmission is there for people who don't have access to manual shifter controls. That's why it simulates throttle lift and gear throw unlike a real automatic which shifts instantly. And why the automatic option still gives you a simulated ingame clutch instead of a transfer case, only that it's automatically controlled.
I do agree however, an automatic option which shifts based on engine load, while it doesn't make much sense in racing, could be a rather simple thing to implement for all the people who enjoy cruising.