Here's one: when your code has no tabs, it's very obvious that whitespace is just a bunch of spaces. With tabs, it's harder to say, especially when mixed. It's one less thing to think about.
My editor (VSCode) notifies me if I open a file that has any mixed tabs and spaces and offers to fix them all with one click.
> no way in hell am I going to change Vim to display them differently!
Sounds like we've found the problem.