Action Economy in Fifth Edition - Targets in Area of Effect

The intended purpose of targets in area of effect is to adjudicate how many targets are hit by an area effect spell. So for example, if a monster has the ability to cast a fireball, it becomes worth using at four targets. If you can get four targets in that spherical area of effect, the monster will use it. Time is time is money in fifth edition. An action economy in fifth edition is a really, really big deal. Some of them have bonus actions or reactions, but not all of them,. which makes bonus actions and reactions very valuable.