There are some programs (especially on Linux) where they don’t distribute compiled binaries and you just download and compile the source code yourself to be able to use the software. This can be because of legal reasons, technological reasons, or even just because a developer wants to be very transparent in what’s being run on your machine.

This is especially common with software alphas (either for new software, or for testing updates to existing software) where they just don’t bother compiling it for every type of system when it’s really just for use by a handful of developers while they’re actively working on or testing the code.