Truly reusable components are clearly separated from the code that uses them. That makes it easy to use the component in a different project. It also makes it easy to rip the component out and replace it by a different one. Non-reusable components cannot be used in different projects. At the same time, they tend to be hairy and almost impossible to rip out of their native project. They are interconnected with the rest of the codebase in subtle ways. Replacing them can break the functionality in unexpected manners and even if it does not you can never be sure that there's no hidden problem waiting for the most inconvenient time to strike you.