I need it all the time. A very common case is caching of (expensive) computations. Let's say you have a parameter for an audio plugin and every parameter change requires some non-trivial and possibly expensive computation (e.g. calculation of filter coefficients). To avoid wasting CPU cycles on every audio sample, you only do the (re)calculation when the parameter has actually changed:

  double freq = getInput(0);
  if (freq != mLastFreq) {
    calculateCoefficients(freq);
    mLastFreq = freq;
  }