The effects of some interventions are obvious. For example, it is obvious that using a parachute is a good way to avoid dying when you jump out of a plane. However, the effects of most interventions, for example changing a farm management practice, are not that obvious. That is why we need fair comparisons, for example comparing what happens when a farmer changes to a new weed control method to those that do not.
REMEMBER: Few interventions have such obvious effects that we do not need fair comparisons to know how helpful they are.