Apple - What are the implications of "Select Your Country or Region"?

You can change this setting later at any time.

It doesn't affect the AppStore. The region setting will affect the availability of certain features which aren't available in all regions yet such as Apple Pay and Apple News.

Apple News is only available if you set your region to Australia, UK or US. Apple Pay is additionally available in Canada.

It will affect the time format (12 vs 24 hour), date format, currency display, temperature unit, and number formatting,. You can change later in Settings -> General -> Language and Region.