After Audiobus appeared in AppStore we received the possibility to route audio between music apps. And while you're running several music apps at the same time you may want to control it with midi - it can be just the midi keyboard, midi arpeggiator, sequencer or controller or such devices like iConnect midi with tons of midi ports. So we need midi routing, and there's an app for that.
Here's MIDI Bridge. It identifies MIDI ins and outs and let you link your routes. Want to connect non-compliant with core midi Akai Synthstation to core midi app? Just link it.
Midi Bridge also allows to make some deep settings and complex midi setups: route particular channel to the app, transpose notes, replace channels, filter events, etc.
See few screenshots here to have better understanding on the settings available: