Even as powerful as Access is with VBA as its native language, limitations exist. Although Microsoft has been nailing those limitations with every version of the product the company puts out, shortcomings still can be seen. API calls are a way to handle those shortcomings.
Dynamic link libraries allow you to call routines from sources other than Access, and they don't have to be recompiled when the application is. There are ways that you can use the routines from these libraries that are totally invisible to users, although some issues need to be dealt with concerning the understanding of built-in and user-defined variables.