Since I'm an American, what's the need for me to update?

What update are you referring to? If you mean iOS 6.1.3, there are a number of security updates that apply to all iOS devices:

http://support.apple.com/kb/HT5704

If that's not what you mean, please explain further.

Regards.

You probably aren't going to use the Japan Maps but Its a good idea to have the Maps application with the most up to date Map information.. The other bug fixes on the other hand are important.

The update for 6.1.3 addresses a security loop hole that allows a bypass around the passcode lock feature of iOS devices (ipad,iphone,ipod) so i would recommend the update, Apple also just happened to include an update to apple maps in this update.