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It's a common thing in the electronic music industry to release stems as a part of a remix contest. Dozens of remixes get uploaded to Soundcloud, the authors of the original track cherrypick a couple of them, package them into a "(Remixes)" EP and release it on their label.

It's like an initiation step for bedroom producers. You build up your reputation by having a couple of remixes released by an actual label, your remix gets played by DJs associated with that label, you start to develop relationships with the industry people in the process... It's an entire ecosystem based around free stems.

This on the other hand is aimed at DJs who use vinyls to layer on top of different drums, but it's the same principle. They want their tracks to be sampleable to other artists.