Like you, I'd thought I had a good run without it.
More recently, I noticed that some of the people who were way junior to me earlier in my career, are way ahead of me because they were strategically switching jobs every 1-2 years. After speaking to some of them, the common factor turned out to be LinkedIn, and I begrudgingly rejoined it in January.
I just recently got a new job that I think it's pretty good, and I didn't have to submit a single "blind" application. All of my interviews came through recruiters via Linkedin.
It's a relatively low effort way to have a steady flow of opportunities, one of which might turn out to be good once in a while.
I do very actively maintain my linkedin page, but honestly I got all of my jobs by personally reaching out to companies where I want to work (through their own website).
As such, I wonder if this is correlation vs causation: people are really eager to advance their career might want to keep all options open, including linkedin, regardless of whether it has actually helped them in their career so far.