When i was pregnant with my son, the doc had me coming in 2times a week to check my levels. They probably want to make sure the hormone levels are rising like they're suppose to. There are 2 different pregnancy tests. One blood test tells you if you're pregnant, and the other blood test tells you how far along you are. They could also be testing you to find out how far along you are.
I'm no doctor, but I do know certain hormone levels increase rapidly after conception. So maybe by seeing where ur hcg levels are will help them better detetmine how far along you are.