I think there are a variety of reasons.
A study has been done on this. "Likes" and "follows" translate into approval from others. Some people are seeking approval, others not so much. However, "follows" on Twitter, for example, mostly mean your message reaches more people, the ultimate goal.
Some are looking for the latest news about critical issues. They also want to know what others think of those issues.
Some pass on information they think others should know.
Some are lonely.
Bored with their own lives.
Just as many are addicted to their cellphones, some are addicted to social media.
Some find pleasure in attacking others who say things they don't like. There are organized groups who do this to certain people. They are trying to diminish those people in the eyes of others.
Some think social media is fun. They learn things they didn't know before.
Some enjoy discussing their favorite subjects and hobbies with others.
Some use social media to share their experiences. They might not have anyone to talk to or anyone who will listen to them at home. I have learned a lot from them.
Some ask for advice because they are stuck and it makes sense to them to get a variety of opinions before making decisions.
Some ask for prayers for a loved one, human or pet...well, that doesn't need to be explained, does it?
There are more reasons, but this is a start.