They tricked you into giving them access to your email. They used endorsements to trick you into logging back in (and hitting the previous trick login...). The marketing bods who came up with that shit will occupy a special place in hell along with every other growth hacker.
The content is about as low value as on any other social network but compared to the toxic waste Twitter and FB spew out, the worst you'll see on LinkedIn is an annoying writing style where everyone seems to be drafting a motivational seminar or self help book.