When Google first introduced the ability to attach files from Drive directly in Gmail, it originally only supported sharing links. Now, you can attach the files directly.
The change helps Google Drive become even more useful as a file storage service (as opposed to a fancy folder for Docs and Slides files). Of course, using this option will make a copy of a file, rather than giving access to a shared one that everyone can edit at the same time. But it's a welcome change for those times you need to give out individual copies.