I think these people fall into 1 of 2 groups, which explains the "why":
1. They're trying to be a "thought leader" on a particular topic (to get more/better job offers in the future).
2. To grow awareness of a brand.
In a lot of cases they're probably doing more harm than good, but in some cases it is a cheap and easy way to get a lot of attention (which is also why it bothers us, and that is how they're doing harm).
I use a lot of discretion when I post on LinkedIn, 100% on brand.
But I really do get a lot of support from having an opinion or posting a perspective on a nuanced topic.
It basically took me coming to terms with not expecting to work for anybody anymore. I am not worried about recruiters and hiring managers taking a dim view of opinions.
But other movers and shakers really do listen, its made it easier to get into rooms, get capital, get acquirers, get legal and the attention of people I want and also didn't know I wanted.
It is a positive feedback loop and I know plenty of employees with opinions are not speaking or liking publicly. Its very similar to following sexy instagram accounts but you don't like or comment anything because your significant other and their friends and potential other mates will take a dim view on what you say. An employer/employee relationship is the same way and describes LinkedIn behavior.
I agree, and I follow a few people who do it well and I get value out of following them. But, my comment about the people doing harm to themselves (or their brand) are the people who are bothersome and are posting what amounts to spam. It doesn't sound like you're in that camp.
I think these people fall into 1 of 2 groups, which explains the "why":
1. They're trying to be a "thought leader" on a particular topic (to get more/better job offers in the future).
2. To grow awareness of a brand.
In a lot of cases they're probably doing more harm than good, but in some cases it is a cheap and easy way to get a lot of attention (which is also why it bothers us, and that is how they're doing harm).