They also trace your mitochondrial DNA and Y chromosome so you can see where your maternal and paternal lines originated, if you're interested in that sort of thing.
The part you're referring to is probably the disease risk section, where they show you your comparative risk of various diseases compared to the population. This is simply a better resolution version of your family history of disease, which a doctor will ask you if you ever go to one. Are doctors fortune tellers?
23andme is expensive infotainment without the required education in both the genetic and the mathematical/statistical background.
I can see what you're saying, that if you're a qualified doctor, or have studied medicine for years, then 23andme could be useful, but for everyone else, I think it could be very dangerous.
I don't know how things are in the US (I know health care is 'interesting' over there), but one of the jobs of a doctor is to take information and interpret those results for the patient. That's why they spend years training.
Giving patients access to tests directly doesn't seem a good idea at all to me. It's like showing a windows user a stack trace when a program crashes. Most of them won't have any clue how to interpret that stack trace, they'll probably get over-worried their computer is broken or has a virus, etc etc.
Doctors are a fine filter for stupid people, but it they can be infuriating for people like us. My mother is a biochemical engineer, and when she was pregnant with me she was so frustrated by the doctor's dissembling over her amnio, she grabbed the test results out of his hands and read them for herself.
I'm regularly frustrated with the lack of doctors' honesty. I don't need one to decide what I do and do not need to know about myself. I think we not only do we need more direct-to-consumer medical testing, I think we should have better access to our own medical information.
But, it doesn't really give actionable results, especially if you're young and healthy.
But mostly this is awesome because the future is here!
Most of the stuff listed on the 23andme site is just looking at slight correlations and assuming causation. (Though there are some major correlations that strongly suggest causation.)
I like it b/c it is helping me learn about genome research in an interesting way, and it's preparing me to be ready to digest the results of a full genome, another few decades of research, etc.
Plus, the genealogy stuff is pretty interesting.