On the Symfony blog they've made a post about a change in all Symfony versions around how .env files are handled and what prompted the change.

When Symfony 4.0 was released, the .env file was introduced as a way to set environment variables. The core of the system has not changed. But, thanks to recent updates to some core Symfony recipes, .env loading has some new features that can be enjoyed on any Symfony Flex project!

If you have an existing Symfony app (started before today), your app does not require any changes to keep working. But, if/when you are ready to take advantage of these improvements, you will need to make a few small updates.

The post outlines what changed exactly including the removal of the .env.dist, allowing a .env.local to override the settings and that the .env file is now pulled in for testing. The final point is the main reasoning for the changes to make testing much easier when it relies on these environment variable values. It doesn't require any changes to your current application but can be optionally implemented to take advantage of these updates.