Mohamed Said explains the not so well known sometimes validation rule in Laravel.

Laravel's validation library is very powerful and easy to use, using a few keystrokes you can build a strong defence around your application, preventing invalid user input from corrupting the application flow and potentially introducing bugs. ... In this post I'd like to highlight a trick related to conditionally adding validation rules.

http://themsaid.com/laravel-advanced-validation-conditionally-adding-rules-20170110/

To learn more read the relevant section in the Laravel docs.