I use music theory (i follow the "circle of fifths" and some harmonic exceptions) to select which song follows, to create a perfect dj mix that sounds synched in tempo and harmonic in key.
No more key clashing funny mixes . Only bpm matching is the half of dj's job. This is why house producers makes music dj friendly. That term means adding bars of drum loops (no instruments with notes) in the begging and the final part of the song to allow djs makes mixes without clashing harmonically.