Indeed,

All implementations will either be coded from scratch or heavily modified from their original implementations.

Sometimes you are just reinventing the wheel by coding from scratch. Any seasoned developer will tell you to utilize existing code if its available and fit for purpose.

A great example is Encrypted Messaging, most coins share the same Encrypted messaging code. It works and does what its supposed to but there are known flaws with this codebase. In this instance, we would look at the original implementation fix any bugs then improve and streamline the code. Adding additional features and building on the code base.