In D&D 4E as with many RPGs, characters have a fixed armour value or AC. When attacking, the attacker must roll a die to overcome the target's AC. In 4E offensive magic works the same way: the target has a fixed Fortitude/Reflex/Will defence determined by their stats, and in order to hit, the attacker rolls to overcome whichever defence is relevant.
However, it was not always like this. Some spells in 3E and now again in 5E work the opposite way. For those spells the target is the one who rolls against a fixed value determined by the caster's stats, in order to avoid spell-damage.
I understand that many features of 4E were not popular among fans of earlier editions. For example how a straightforward skirmish can take hours at the table. But I have never heard any criticism of introducing Fort/Ref/Will defenses. I personally like attacks and spells working the same way because it simplifies combat while still accounting for the stats of attacker and target, and has an element of randomness. What was the reason saving throws for spells returned, other than reverting to tradition?
I would like to know why the designers claim to have made the switch—not an analysis of the benefits and drawbacks—unless indeed those benefits and drawbacks are cited by the designers.