Never really thought about it before, but... Why does armor scale?

Mostly excluding tanks from this, because it sort of makes sense for both armor and magic resist to scale on them. So yea. Was there a design reason behind it, or was it more just one of those oddities that came over from DotA? EDIT: Specifically referencing the innate scaling with level, not scaling in general.
Report as:
Offensive Spam Harassment Incorrect Board