I found the job I have now because I was foundering and a college friend put me in touch with a recruiter who was a good match. Now that I've worked in industry for a while, I have colleagues who've moved to other companies I can ask them about. Sometimes old bosses will get in touch and ask me if I'm interested in joining their companies. The more positive experiences people have had working with you the more doors are open.
Indeed, one of my bigger customers now was in a junior role at a company I did some work for years ago. You never really know where opportunities might lie and it costs little to keep in touch.
I think people also get too shy about reaching out to people they haven't been in touch with. Most people aren't going to be annoyed if you say, "Hey, long time no see. How have you been? Wondering if you know anyone who's hiring" or whatever it is you're asking for.
How? Let's say I work at some company...Then I know exactly my coworkers and customers. How does that help me build a network?
I could maybe ring up some buddys from uni that are working somewhere else... But contact to them also gets lost over time.