The reusibility depends on the function itself. When you have a lot of system calls or basically you use 4test language, functions in a .inc file is the best. You can decear a .dll within those file and do system call or use API methods. If for some application and window declearation specific testing, you can write function to do those repeated work. you cannot call it if it is a testcase. this kind of function maybe not portable as the first kind, but it still better than testcase.
I don't know how come you people are comparing a function with a testcase?A testcase is a container for function as well as structured code.Now,whether you include the function in a .inc file;write in testcase and call it through through main() or write the code in structured manner,its the code that is manipulated through testcase.The difference is basically there.A function can be run,and most imp.,there is a defaultbasestate for a function so appl. can be run by itself.
If it can clear your concepts,my purpose will be solved.