First, what is Localization? What does that mean? For App Developers, Localization means making sure that your source code does not contain any static localized representation of strings and data. Instead, these concepts are outsourced to strings files or the system's components that are based around the user's locale.
This one should be pretty obvious! The more locales you support, the wider your audience becomes.
If you work for (or contract for) a government you may be required to support the dominant languages in the region. For example, in Houston where I live I might be required to support English, Spanish, Chinese, and possibly Vietnamese.
Let's be honest. Not everyone speaks English. Not everyone wants to use an app that isn't in their primary language. We seek great UX for our users, and language and region accommodations show respect for your users.
This goes beyond just language.