Levels

I just got a bfp (yay!) I keep seeing people say what their HCG and progesterone levels are and saying how much they are increasing, but my doctor assured me that I should wait until I'm 7 weeks to come in for my first appointment. It's my first pregnancy, but if course I'm worried about all of the potential negative things that could go wrong. How come some get these levels tested early, but my doctor says I don't need to?