Not just because of SEO, although that's one reason. Logged in users are already sold on LinkedIn, so LinkedIn wants to encourage them to make more connections.

Not just because of SEO, although that's one reason. Logged in users are already sold on LinkedIn, so LinkedIn wants to encourage them to make more connections. The aim for guest visitors is to get them to sign-up, so showing them a more useful page is a better way of showing what value LinkedIn can offer them.