Using the function
Now that you’ve tested the function on problems for which you know the right answer,
you have confidence that it works correctly. So you can use it on problems for which you
don’t know the right answer, and be reasonably confident that the answers it produces
are right.
Just as important, you can now use your new function in writing other functions,
knowing that if something is wrong with the new function, it’s not the old function’s
fault. When a new piece of commercial software is written, it may rely on hundreds or
thousands of previously-written functions; if a programmer had to re-examine every one
of them to fix a bug in the new program, nothing would ever get done.