They could get to know you in all of those ways, networking obviously gets your name out there more but to be honest it's mainly blind luck that someone you worked with has a job opportunity.
My boss at my last job used to hold CVs up in the office and ask if anyone knew the person on the CV. If the answer was no they usually went in the bin. If the answer was yes then he would ask about what the person was like and, if the answer was good, they would probably get an interview. This is a pretty extreme case (and technically illegal) but it highlights the point that most employers (including academics) would prefer to employ someone that they know. Some people look very good on CVs and interview well, but then turn out to be @rse holes. If you know them already then you know that that won't happen.
Incidentally, I got my next job because I knew a few people who worked at a different place, in fact my current job is the first I've had where I haven't known anyone there before I started.