What is a midi controller ?
We had to start with this question,
as there are still a fair amount of users that ask why the iTM apps don´t produce ay sound on their own...
Wikipedia: "a hardware or software which generates and transmits MIDI data to MIDI-enabled devices"
This basically means that a midi controller does not produce any sound !
it is just a way to control / interface with any kind of midi enabled software or hardware
As you can probably understand a little better now
none of the iTM apps are able to produce any sound but if you use them to control a soft synth (for example )
they will then allow you to play it thru their UI (user interface)
You can use iTM apps to control not only soft synths but any software that is midi enabled
In audio world this means software and hardware synths and fx, daw control, etc
Midi is also extensively used to control different applications dedicated to VJing, Video editing, among many others...