We've been doing something a lot like this for as long as I can remember.
Back in the 1990s if there were any big unused copper areas on your PCB you'd mask them to save on etching acid - a gallon of acid would have a lifetime measured in square inches of copper removed, and the less copper you removed, the longer your acid would last.
Meanwhile, a lot of DIY etching processes were very basic. Sure, you could get translucent acid and a transparent bath and heat it to a controlled temperature and run bubbles through it and so on. But if you were on a budget, some room temperature ferric chloride in an old ice cream container would get the job done. And getting the etch resist onto the board? You could draw it by hand with special pens, use transfers, there were special printer toner transfer papers, or you could DIY UV photoresist using printable projector transparencies and the sun as your UV source.
This was not a super-scientific, tightly controlled process.
If you had narrow traces and narrow gaps on one part of your PCB, and large areas of copper to remove on another? Well, if you left it in the acid long enough to remove that large area, could be the narrow traces get etched away too.
So masking off any large areas meant all the copper getting etched was about the same width - thus compensating for the poorly controlled etching process.
Of course, these days professional PCB manufacturing is orders of magnitude cheaper than it used to be. When you send your design to pcbway or jlcpcb they have much tighter control over the process, so you no longer have to worry about this stuff.