Now that I have an APC 40 I don't have much use for this. But it's still pretty cool and has been how I do things on stage for years now.
Essentially, most electronic/laptop musicians dream of never having to actually look at the laptop, or touch it during a performance. There's various external controllers to do that but most focus on playing keyboards or samples, or changing knob values. By using the Mackie Control Protocol (without even owning any Mackie gear!) you can actually use those controllers to get access to a ton of different functionality in your DAW.
In Ableton Live for example, I use 4 buttons on my Trigger Finger to navigate left, right, up, down to find an trigger clips and scenes in session view. This is far cooler than clicking around with a mouse on stage.