You probably have a list of people you know well -- and would reach out to first when you need help (like finding a new job). However, don't overlook those you have less close relationships with. They may be able to provide information that is just as useful.
Kim Keating explains why people you are less close to can often be more helpful:
Both weak and dormant ties offer more novel information than strong ties. They travel in different circles and are connected to entirely different people -- unlike strong ties, who tend to travel in the same circles as you do.
Of course, you don't need to transform these weak ties into stronger ones to build a stronger network -- you just need to keep in enough contact that people remember you (like helping them out without an ulterior motive.)