[NOTE: This ended up becoming longer than I intended, so feel free to not read. Or stop somewhere along the way.]
It's the other way around. "In the beginning", there were only global variables. So any variable used in a macro sat around forever.
Eventually someone realized that having a lot of variables sitting around not being used actually slowed things down some (a small amount, but still). So we started a "best practice" of trying to always "delete" variables after using them.
We bugged @peternlewis to add Local variables, and IIRC, he said something about us not holding our breaths. I could be wrong. And then one day he announced a version with Local and Instance variables, and the world rejoiced. Well, a handful of us, anyway.
So now most of us "old hands" use Local and Instance variables exclusively, unless we really want something to hang around. You might expect to see nothing but "DND" global variables now if I looked at all my variables, but it's hard to go through and change everything.
So that's why I wrote the Variable Inspector Prompt, which lets me sort variables by size, so I can at least find the big ones and change their macros or just delete them.
Sheesh, I can pontificate sometimes, can't I? 