java - Why does Eclipse asks if you want to add a new project to build path?

I need to clear up a misunderstanding I have with regard to Eclipse and how it manages build path. When one creates a new project, Eclipse offers the option to add the project to the build path. I had assumed, until I noticed this, that any new project being developed would automatically be on the build path, otherwise how would one class in a new project compile and run with another class that it depends on in that same new project? (And yet this happens, seemingly by default, whether the initial "add to build path" option is checked or not.)

What am I missing about build path, and why would one need to explicitly add a brand new project to the build path?

Specific way I am creating a vanilla Java project in Eclipse: File > New > Project > [Select Java Project from tree selector]