Hi there and thanks for your question.
The advantages of a virtual, customizable MIDI controller are usually because you need to:
- create a user interface because your target doesn't have one or has a very limited one
- customize a user interface because you would prefer it to be different (same as #1, pretty much), sometimes for a purpose like "performance" or "editing."
- aggregate several targets in one user interface
And if you have a hardware MIDI controller, the question is what it does, and what you want it to do. If it's just a keyboard, adding essentially unlimited knobs, sliders, XY pads, etc., via MIDI Designer is great no matter what you're controlling.
Comment under this answer to continue the discussion, thanks!