I noticed that a lot of GitHub accounts only have repositories which are forked from other accounts. In addition the people who do this usually don't make any contributions to the forked repositories.

I've heard of people collecting stamps and seashells, but why would anybody want to collect repositories? Personally I would only fork a repository if I wanted to make some changes to it.

As you mentioned in your question, people fork repositories when they want to make a change the code, because you
don't have write access to the original repository (unless you've been added as a collaborator by the owner of the repository).

In the forked repository they have write access and can push changes. They may even contribute
back to the original repository using pull requests.

I think there are multiple reasons why people fork repositories but don't change them:


they might fork a repository which looks cool, simply fork it (because it's easy (only one click)) and want
to make a change later (and then probably forget it/didn't have time to do so)
they fork a repository to make a change and then discover that the change is unnecessary and forget to delete
the own repository
they might fork a repository because one of the projects depends on an other repository (maybe via submodules) and
they want total control over the repository used as a dependency (the owners of the original repository might decide to move from github to google code etc.)
they might simply forget to push the commits

