Formatting. It's tougher to render (for instance) source code in a clear, easily understood fashion in a proportional font than in a monospace font. The same

Formatting. It's tougher to render (for instance) source code in a clear, easily understood fashion in a proportional font than in a monospace font. The same goes for a lot of mathematical and scientific writing, cases where red-pen editor's marks are needed, and so on.