Welcome to the Forum, Frops!
You're right, it's not possible to be a self-employed doctor, or detective. If you have GTW, you can sell food in a bakery or café.
I think the styled rooms are a way to showcase the career rewards that you unlock. I don't use the rooms, but my chef's kitchen will usually have the counters and a couple of the appliances. I might set up the doctor's office and put a teapot in it so that my sims in a rotational game can go visit the doctor. And I suppose if you're doing a story, you can set up the detective's office and send sims there for a consultation.