Sure, let me think of a couple of reasons:
1) The network effect of the timeline is HUGE. If people share/like your post, you can have a reach of ten of thousands of people. I scored new customers this way.
2) I often post something about something I've done (new certification, new project I launched, whatever). This almost always leads to replies from connections from unexpected people in my network. The thing I posted might have struck a chord with them, they may see ways for us to collaborate, it might be a product that they need, they may have valuable feedback on it, etc.
3) I interacted with many people over the course of my career. I may have forgotten about those people, but having them all together in an easily searchable "address book" proves pretty valuable to me when I am looking for people with a particular skillset.
4) In the past I did find jobs through linkedin. Either by applying to the job posts or by simply asking around with recruiters.
5) I use LinkedIn a lot to find out who works where, what role they have and who to address when I need something from another company.
There are also counter points to be made:
- I have received a ridiculous amount of recruiter spam over the years. Taking all skills I don't use anymore (like PHP) off of my LinkedIn has certainly helped.
- Advertising on LinkedIn is garbage. We were looking for Dutch speaking candidates in Amsterdam. We got 0 Dutch applicants. All we got is 100+ Indian coders who barely speak proper English.