Counting bytes within an `io.reader` is crucial for efficient file handling in Go, and this process can be accomplished elegantly using the `io.copy` function alongside the `io.Discard` writer. To develop a `countBytes` function, a test-driven development (TDD) approach is leveraged to ensure correctness, starting with defining test cases for various inputs. Instead of manually iterating through bytes or using complex constructs, the `io.copy` function simplifies the counting process by copying data to a writer, which, in this case, does not need to retain the data, allowing the use of a no-operation writer. By implementing and thoroughly testing the function through diverse scenarios—including edge cases and Unicode characters—developers ensure robustness and precision in byte counting, setting the stage for future integration with larger algorithms that require line and word counts.