Many people in indonesia change its country marketplace because apps selection stuff
I heard US is most complete

Read microsoft support page for more detail,
but some important stuff

IMPORTANT Once an account locale is associated to a Windows Live ID, it is permanent and cannot be changed.

To access the Marketplace of a country other than the one in which your account was created or to switch between catalogs in markets where more than one Marketplace is offered, you must reset your phone and complete setup again by using a different Windows Live ID.

Marketplace country is determined ONLY by the locale of the user ID associated with the sign in account.

Marketplace locale is NOT affected by the Country/region setting of the Windows Live ID or the Region+language settings on the phone.

The account locale consists of a country and a language. For example, the locale for the United States is (United States – English).