What are the audio stems and why are they sold separately?

Stems are the separate tracks that make up our tracks (bass, drums, guitars, etc.)
If you purchase an exclusive track (only in that case) you can also purchase as an option the stems that were used to create the song, in case you want to re-mix it or modify it as you wish.
The format of the stems is WAV audio 24 bit and 44.1 khz.