The important thing about reader macros is that they happen at read time. It causes code to be executed at tokenizing time (so you can make your own syntax, e.g. #{ ... } could return a JSON object directly) even before evaluation or macroexpansion.

So #+ and #- really only scratch the surface. Someone wrote a comment about using #+ to define the path delimiter (/ or \), but that is something that could be looked up at runtime. Read-time extension is unusual in programming languages (C++ has such a feature for a more limited set of cases).