While only one user can upload a file to their Flightsim.to account, they can add contributors to their project or file. Contributors are users who have contributed to a project. Contributors can access the file statistics to view downloads, for example. They can also comment on the file and a "Contributor" badge is displayed next to their name. Contributors can also edit the roadmap of a file and create group announcements. Contributors also receive half of the karma points that the author receives.
However, contributors cannot modify the file, edit it, or upload updates. Only the owner of the file can do that.
Essentially, it's about giving decent credit to contributors by displaying them on the project's page.
You can add contributors by modifying your file and then selecting the Additional tab.