Seems like you'd kind of have to have a clear idea of where everything goes before you start to really work effectively. Or is that part of the appeal, practicing skills you normally would use as a coder?
Definitely you should have a clear idea of your diagram before you arrive at your editor. If you want to experiment with layouts or whatever, you do that in a different tool, often pen-and-paper.
As you note, the same is true of programming! Experimentation is a totally different modality than writing a production-quality program. It's not a spectrum, you're either doing one or the other.
edit But, really, if you have a reasonable diagram in your head, it's really not difficult at all to express it as ASCII in a (multi-cursor capable) editor. I can do diagrams faster in VS Code than Monodraw, usually.
I always thought they were kind of like the art you see on silicon chips, not really practical but accepted because trying to only do 100% practical things for a solid 8 hours isn't really the best plan, and only done by the programmers with a talent for raster art in general.
It does seem a bit time consuming unless you have a strong ability to visualize entire layouts. Being able to imagine 3 boxes is a lot different from being able to imagine 3 boxes on a screen all at once in proportion, and that's pretty far outside of what programmers ever are trained for unless you specifically seek out ways to learn that.
Using real pen and paper for diagrams is an interesting idea! Sometimes I see other coders with a notepad on their desk but I never got into it and am never quite sure what it's for, usually people seem to be making a lot of bullet points of text.
