1. It has your resume posted for recruiters to see. You get emails from recruiters scouting you for jobs you might never have found on your own. Many recruiters are useless 3rd-party recruiters, but some are useful 1st-party recruiters.

2. You can keep in contact with your old coworkers; people you aren't close enough to have as friends through some other venue.

3. It doesn't actively spread conspiracy theories and destroy society. You get some recruiter spam, and that's it. The site is nothing more than a tool to aid your professional life and career, and it doesn't try to be anything more than that.

What's the problem, exactly?