[imoverclocked]

You might read up on pure functions [1] which are unable to produce side-effects when run.

Imagine you have two methods in Java:

  int add(int a, int b) {
    log.info("Adding two numbers {} {}", a, b);
    return a + b;
  }

  void doStuff() {
    add(1, 2);
    add(2, 3);
    add(3, 4);
  }

The result of add in doStuff is unused. However, add has a log statement which someone might be relying on elsewhere. The log line makes understanding the usefulness of this code much harder. ie: Can you delete this call? It's impossible to know without understanding everything that might consume the log line. The log-line is a side-effect in these methods.

In languages that understand "pure functions" there are optimizations that can be done by the toolchain (think automatic memoization, deferred computation, and much more) when only pure functions are called.

[1] https://en.wikipedia.org/wiki/Pure_function