Im referring specifically to the FBV MKII shortboard foot controller from Line 6. Everything you can do with the foot controller can be done with the iOS app. The only thing is with the iOS app you've got to stop playing to operate it and the foot controller you just step on it. In the app you can turn on and off effects for a specific patch but with the foot controller you always have the stomp, modulate, delay and reverb buttons you can turn on and off even if its in the specific tone or not. I like using this since with one specific tone I can press delay or stomp or modulate as the song needs it. If the delay for example or the modulator effect is not how I want it I can edit it with the app and then save it. In a live situation its easier for me to bank down and up, press ABCD depending on the tone I want for a specific song and then add delay or take it away for example or go from clean to stomp with a press of the pedal.
I haven't used its capabilities with garageband yet, but Ive read you can control recordings with the foot pedal. All in all, the foot pedal just makes it easier to change effects and tones depending on a song or a part of the song. There is a youtube video about it.