What is SourceForge used for?

SourceForge is used for hosting and downloading open-source software — it provides project pages, file downloads, mirrors and basic dev tools (issue trackers, wikis) for tens of thousands of open-source projects, and it also runs a general business-software directory with reviews.

Historically it was THE home of open source in the 2000s, before GitHub absorbed most active development. Today its main practical uses are: downloading established open-source apps (many older but maintained projects still distribute there), finding mirrors for large binaries, and browsing its software directory.

Worth knowing: during 2013–2016, under previous ownership, SourceForge bundled adware into some installers — that reputation still follows it, though the practice ended after the 2016 acquisition. For active development and newer projects, GitHub/GitLab are where the ecosystem lives now.