Not quite sure why one should use a script when this can be done directly from a MIDI In block using Merge.
What am I missing?
You’re not missing anything. I like having as many things visible and obvious in my workflow as I can. After all, that’s part of the point of a modular visual wiring design, yes? 
In the particular case above, if I used a scriptlet and a channel constrainer, I could immediately see that a MIDI message was coming in, the velocity was being adjusted by the scriptlet, and the channel was being adjusted from 1 to 9 by the constrainer before passing into the instrument.
It’s just a personal preference, really. Obviously the solution from @pianopaul works fine and you can achieve everything I listed in that one MIDI In block. It’s just not as obvious what is happening in that workflow, visually speaking.
In the above advice from @pianopaul, he suggested I’d need to map to two widgets in the MIDI in block, is there a way to keep it to one?
When you only want to scale up the velocity then 1 widget is enough.
But what issue do you have in using 2 widgets?