Actually curious - I've never had any interactions through the site other than randos endorsing me for things I know nothing about, and code-sausage-machine consultants and recruiters sending me emails about positions that don't match anything in my profile. But it must have somebody out there that uses it, if they're pulling in 3.6 billion in revenue.
Now imagine you're a buyer for a large furniture retailer, if you have your profile open, it might not be unusual for a supplier looking to get into somewhere bigger to contact you.
I've worked with a few sales guys, what they tend to do is use linkedin to build a tactical map. They then strategically expand their network until they get close to who they want.
LinkedIn, when used correctly probably won't come in the form of an inMail, but in the form of a phone call out of the blue from an acquaintance.
Also good for finding well respected people in my field, figuring out from that which companies are doing good work there, and keeping tabs on the companies so that I have a clue what the more global picture is outside of my immediate circles.
I do not work in programming though.
I work in tech.
As someone else mentioned, I update it when I'm looking for a job. That kicks off a bunch of messages and invites from recruiters.
There's the usual hits and misses, but it works fine for me.
It's funny you mention that because I feel like in the last year or so LinkedIn recruiters have caught on a bit. I've received far less solicitations than years past (at one point I was getting 5-10 per day). In fact I rarely ever get LinkedIn spam these days. Instead I'm seeing more spam from Dribbble and AngleList.