What is the purpose of followers?

Well I guess it is reciprocal not sure.

it works for me, so I can see their activities, discussions, responses and comments.

You can easily check discussions by followers in one page. It's convenient. That's it.

Following is useful for not to miss a single post from our favourite people.

Not always, but a lot of the time I notice people follow that never show up on m posts, which seems out of place. I think some people think if they simply follow you, you will mosey on over to their posts and they won't have to do much else. I don't actually follow anyone. I respond to those who are active on my posts, and I troll the most recent tabs to find people and posts to read. I never use followers to find people or things to read.

It does help you not to miss the posts and activities of them. So if you like someone's post, you can follow them to not miss any of their posts.