Self-motivated people can work too much. A job title can help remind them where to stop, and can provide support to that person in arguing why to not take on more work.
Job titles provide a baseline reference for the outside world. That was a lousy description, so I'll give examples.
If you apply for a loan, they'll want to know your job title. A job title which reflects one's position and authority in a company is a proxy for both current income and expected career path; information the loan company uses to help make an offer.
If you apply for a new job, then the job titles of your previous work act as a summary of what you did, and if you did more than that then you can highlight them in the description.
(And remember, some companies may call previous places of employment to verify job title and dates employed, so having an accurate title is important.)