They are invisible whitespace characters that are handled reliably and flexibly in every editor that I've ever used them in. The same cannot be said for spaces. If a codebase is using two space indenting, I get to look at the resulting puke (this is subjective, of course) on my screen. I might be able to do some hackjob fix to change how they're displayed, but why should I have to? There's a character that satisfies both me and the animal that wanted to indent their code with 2 spaces.
