Guessing here, but +/- is good for describing them as binary opposites as that system already exists. This is a good thing assuming there are two types of charge/pole which behave in opposite ways (Eg move differently in a field). It’s also just good to use numbers so that we can describe the amount of + and the amount of -, which numbers already do. It also allows us to describe neutral as neither + nor -, but 0. Again, we already have a scaffold there for numbers and it’s easy to copy it for new things when that makes sense.