It is very helpful when creating new code to have a model that works and that we can study (or clone). This is especially true in Business Central, where there is little or no development documentation available for many of the different functions we would like to use. One of the more challenging aspects of learning to develop in the Business Central environment is learning how to handle issues in the Business Central way. Learning the Business Central way is very beneficial because then our code works better, is easier to maintain, and easier to upgrade. There is no better place to learn the strengths and subtle features of the product than to study the code written by the developers who are part of the inner circle of Business Central creation.
Function models to review and use
If there is a choice, don't add custom functions to the standard Business Central codeunits. Well segregated customizations in clearly identified custom objects make both maintenance and upgrades easier...