Depends where you live. I can give you the San Francisco Bay Area use of linkedin

1. sharing linkedin accounts is more natural than sharing FB accounts here. And I'm talking about social events, friends, meeting someone new. Not about formal stuff... It's really your social profile here. The rest people don't really care anymore since it's all about networking

2. Looking for jobs. If you're looking for jobs and you're not on linkedin then you're invisible in the Bay Area. Plus, all companies career websites ask for your linkedin profile

3. You can lie to a lot of people about your career. Linkedin acts as your professional ID too.

I use it for networking with recruiters. I get to see jobs that are posted from 50+ people (I have a small network) and get to decide what I want to reach out for. I was able to land a gig at a clearing company because I saw a post that a recruiter was looking for developers for a trading company and I wanted to break into the industry but didn't have enough experience in what they were looking for. I reached out and they found a job that worked for me.