MIDI FX

Buy once and get the iOS and Mac apps (with Family Sharing)
Control your MIDI signals and process them in any combination of ways: Harmonize, Transpose, Arpeggiate, etc... Or use the Sequencer to play any external or virtual synth. Use a MIDI interface to unlock the power of the app to control multiple synths with one keyboard, play one note and generate chords, basslines, etc... Use the delay/looper or filter notes based on velocity or other parameters to send to different synths/effects.
A MIDI workstation in your pocket (or your iPad/Mac)
You can also use some internal instruments (drums sampler, piano or synth) if you are away from your MIDI gear, and sequence whole songs inside the app, or use other synth apps as input/output for MIDI FX. Your imagination is the limit!