Why declare variables

Sorry this is such a general question. I hope this is the right place for it.
I know that we should declare all variables at the beginning of a subroutine, in fact I'm told it's good practice to use Option explicit to 'force' variables to be declared, my question is why?
If I don't declare a variable the routine still seems to work OK so what is the downside of not declaring them upfront? Is it just for neatness or common practice or is there another reason?