This spread is usually used to get a quick reading based on past events, the present situation or surroundings, and what can be expected in the future. By only using the 3 cards to read the past, present, and future, reading only 1 card per time span keeps things simpler, less information to process, and for me, this is much less confusing than 10 or more cards in a reading.  Especially when you are looking for a simple answer, or have time constraints.