There are two ways that spells are rolled for in D&D 5e, depending on the spell: attack rolls rolled by the players and saving throws rolled by the DM.
But as far as I can see, saving throws could be made as attack rolls instead. I know D&D 4e did it this way, with players making attacks against AC/Fortitude/Reflex/Will Defenses instead of the DM making Fort/Ref/Will saving throws and players making attack rolls against AC.
I don't really see why the designers of D&D 5e would choose to have two different ways to roll something that, as far as I can tell, could be simplified into one mechanic (and I have asked about their stated reasons in a separate question), so I wondered if I'm missing something important that distinguishes the two methods.
I guess it boils down to: Is there any difference in probabilities or practical concerns between one or the other method, or are they interchangeable?
This is related to Why do attacks function differently to spells? and Why did saving throws return in D&D 5E? This question is focused on if there is any practical difference, regardless of why they actually made that design decision.