I was a very early adopter of LinkedIn and have found it invaluable for keeping track of my industry connections, understanding who works where, and occasionally for developing prospects or for considering employment opportunities. I have subscribed to the premium services at different times and find them useful when I'm recruiting or am looking for my own role. Because we have such a mobile professional community, I often lose track of where someone has gone. Using LinkedIn in prepping for HIMSS18, for example, was essential. I was able to confirm who was still working at the vendors I needed to interact with and who had moved on.
I track what other interesting companies are doing including monitoring competitive vendors.
I was unexpectedly thrown into the job search process at age 58. I found most of the job openings I pursued through Indeed.com, but researched who I knew, and friends of friends, at those companies through LinkedIn. I was often able to find the person who held the job that was now open by searching for the company name plus some of the key words from the job posting. In some cases, identifying the predecessor would have been useful for getting a read on what was happening at the company. I also used LinkedIn to figure out what kinds of technologies and processes a company was using. That was useful when making a job submission because I knew more about them, what technologies they were using, and how they described them. I would discover this just by reading the profiles of current and former employees in IT at the company (I'm an IT guy). While I was job-hunting it was invaluable. I use it less now that I'm happily landed at a new firm.
I use it to find the right contact at other companies. For example, I used it recently to find an interop manager at a major hospital system. If I had used the Contact Us page on the hospitals website, I would have ended up in a blackhole with no guarantee of a response. Using LinkedIn, I found the right person in less than an hour and we were able to exchange emails the same day.
I look at LinkedIn to get background on sales people who contact me.