I'm sure it varies a lot by type of job, but in my field it is not the first or even fifth place I would look for a new job. There's a lot of noise and postings can often be stale or in some cases are missing details, maybe because they were scraped from other job boards. If you have a large network, you may hear about upcoming jobs directly from colleagues' posts, but I think the general job search features on LinkedIn are pretty useless.

That said, I did learn some time ago that you can use various search operators (Boolean, parentheses, and quotes) which make it a little easier to actually get relevant job search results.