Functions are modularized pieces of code defined as part of your application, or in a library. If a function is written by another vendor it is important that your code calls the function in the way intended by the vendor. This means understanding the calling convention used and how it affects the stack.
When you call a function, the compiler will create a stack frame for the new function call and it will push items on to the stack. The data put on the stack depends on your compiler and whether the code is compiled for the debug or release build; however, in general there will be information about the parameters passed to the function, the return address (the address after the function call), and the automatic variables allocated in the function.
This means that, when you make a function call at runtime, there will be a memory overhead and performance overhead from creating the stack frame before the function runs, and a performance overhead in cleaning up, after...