Why an early US by RE?

My RE scheduled my first US and appointment for Monday and I'll only be 5+3. I conceived naturally and my hCG and progesterone levels are great with the help on 200mg progesterone/day (appointment was scheduled before we had first beta test results back).
From what I've read, there's a good chance we'll see nothing other than a sac at this appointment, so why am I coming in so early? I've been trying to google this for two weeks and I have no answers. This is my third pregnancy (no children) but first pregnancy working with a RE. What should I expect to see and happen at this appointment?