Laravel app.js and app.css why need to use ? Can we removed those?

I am new learner on Laravel.

I am using Laravel 5.3.But I want Use Bootstrap css and Js. But Laravel have default app.css and app.js. Hare app.css actually Bootstrap CSS and also app.js work like Bootstarp js. But can i delete app.js and app.css and you my custom version Bootstrap CSS & JS ?

So I need to know why we will be use this two files ?

via Chebli Mohamed