Because there is a Stylesheet called main.css with 5000 lines, and somewhere in there is a block like this:

  .InnerWrapper {
    padding: 3px 7px;
    margin-left: -9px;
  }

The design just got a makeover, and you’re tasked with updating all buttons to have a wider spacing.

Good luck catching everything, including this class and its usages, without manually reviewing everything that is used like a button, inspecting the HTML, then going over the classes, one by one, without breaking some unintended side effect.