The importance of locales is that your environment/os can provide formatting functionality for all installed locales even if you don’t know about them when you write your application. My Windows 7 system has 211 locales installed (listed below), so you wouldn’t likely write any custom code or translation specific to this many locales.
Edit: The original list of locales has been edited to add additional locales that were not included before. Now 228 listed.
The most important thing for various versions of English is in formatting numbers and dates. Other differences are significant to the extent that you want and able to cater to specific variations.
af-ZA am-ET ar-AE ar-BH ar-DZ ar-EG ar-IQ ar-JO ar-KW ar-LB ar-LY ar-MA arn-CL ar-OM ar-QA ar-SA ar-SD ar-SY ar-TN ar-YE as-IN az-az az-Cyrl-AZ az-Latn-AZ ba-RU be-BY bg-BG bn-BD bn-IN bo-CN br-FR bs-Cyrl-BA bs-Latn-BA ca-ES co-FR