The php artisan key:generate - This is a command that sets the APP_KEY value in your .env file. If you use a version control system like git to manage your project for development, calling git push ... will push a copy of your Laravel project to wherever it is going, but will not include your .env file. So if another developer clones your project using git clone ... they will have to manually enter php artisan key:generate for their app to function correctly.
I hope this helps you.