These stems are here for people that want to pick apart the music to use for remixes and other creatives. Mixer stems are the individual tracks from a composition, this allows for easier access to instrumentals and vocals. In an interview about 'The Stems' release Pronobozo reveals the motives:
"With over 10,000 videos on YouTube using our music and 600,000+ downloads from our website alone, I wanted to make sure those content creators and listeners were automatically protected under a Creative Commons (CC) license which would not only allow them to share, remix, and adapt the music but would also allow them to use it commercially. 'The Stems' let us go a few steps further by providing the individual mixer tracks from all our songs in our entire catalogue, this in combination with the CC licenses, we hope, sets a new precedent to the current trends of openness."
If you are to use this music you MUST use this same or similar license. Please read it.