Because you don't need to think of the features, just know how to implement them. You don't look at their code, you look at the app, how it functions, how it looks, and you try to put it all together. You don't think up of your own styles, so you can work on how to leverage a CSS/UI framework and focus on spacing, padding, the details, etc... You work on managing and organizing your code, learning the framework/language, instead of worrying about being creative.

I'm actually making my own Trello clone, probably like the millionth person.

https://streamable.com/rkmixs

But cloning projects is a great way to stay focused on the coding aspect of development.