A Mi account is required for using bundled services
Various services and apps require a Mi account. For example, buying new themes requires spending Mi currency that can only be obtained if a Mi account is available.
Some bundled services are available only in Chinese
Xiaomi, the developers of MIUI are Chinese and their primary market is China. As such, primary language for most apps and services is Chinese and not all of them have yet been translated to English or any other language.