The basic idea behind networks is something everyone learned in kindergarten or even preschool—sharing. Networks are about sharing. It wouldn't make any sense for you to create a home network if you had only a single PC (and no other network devices) because then there would be nothing to share.
It doesn't matter if you are a single person with multiple PCs or if you live in a house full of people, because the idea is the same. A network makes it far easier to share the assets and resources that are spread across the network.