If your Power Apps solution has any environment-specific configuration items, then an Environment Variable in the Solution gives you a configurable input parameter. Environment variables avoid hardcoding configuration information and having to keep track of and change configuration data when importing a solution.
Some of the benefits of using environment variables are:
No need to manually edit configurable values in a production environment.
Configure one or more variables in one place and reference like a parameter across multiple solution components.
Enter different values while importing solutions to other environments.
Update values without a code change.
Granular level security managed by Common Data Service.
Unlimited number of variables (max solution size is 29 MB).
Service the definitions and the values independently or together.
Supported by Solution Packager and DevOps tools enable continuous integration and continuous delivery (CI/CD).
Support for localization.
Can be used to control feature flags and other application settings.
Figure: Environment variable make configuration information easy
More information here: https://docs.microsoft.com/en-us/powerapps/maker/common-data-service/environmentvariables?WT.mc_id=DP-MVP-33518