I don't understand the Freelancer career.

All it does it give your sim a job title, nothing more, nothing less. This way if your sim works at home, say as a painter, a gardener, a writer, or the many other ways a sim can earn money without going into a job and working to earn their money. This way they won't be labeled as unemployed.

Sorry, I just reread your post again, misread it the first time. The only upside to those careers is that they will unlock certain items in buy mode. If there is anything else I can't think of what those would be.