Since Drupal is open source software, anyone can create their own modules, and do things to the Drupal codebase. When looking at Drupal's development you will see that cooperation works better than each and every person working on their own. One of the ways to facilitate cooperation in the Drupal world is the ability to create project pages for modules at drupal.org, Drupal's main hub.
The project pages contain information about module features, documentation links, and issue queues for giving feedback to module maintainers, report bugs, submit patches, and a lot of other stuff. Having a single site for contributed modules, contrib modules in Drupal lingo, has a positive effect on the Drupal community, as it helps Drupalistas to cooperate even more.
TIP: Modules only constitute one of the project types available at drupal.org. There are also themes, installation profiles, documentation and translations. Drupal core itself is run as a project on drupal.org, using the project name drupal.