For me, the most useful case is when hiring. I usually look up a candidate on linkedin. I can see if their work history for public consumption is similar to the one which is on their resume (tailored for the position). I can also see if anyone I know has worked with them before and then get an off the record assessment of their previous work and a feel for them as a person. Neither of those directly contributes to a hire/no-hire decision but it does help with getting to know what the candidate is like. As a side note, I tried posting a job recently and was hugely underwhelmed with both the quality and quantity of applicants.