Relevant: Does Indomitable let you reroll Saving Throws with Advantage?
There's a line that comes up a lot in class abilities that seems out of place, and I don't know what the difference is between it and using the established "Advantage" in fifth edition.
It usually says something like this:
If you fail a saving throw, you can reroll it, and you must use the new roll.
Compared to (Dis)Advantage which is
Use the higher of the two rolls if you have advantage, and use the lower roll if you have disadvantage
Based on the linked post, I have found out that these do not stack.
What's the difference between the first quote and just having advantage for that saving throw roll?
Best Answer
Advantage and rerolling are statistically equivalent, except for interaction with each other and a few specific game mechanics.
For many rolls you only need to roll the target number or higher to succeed, and it the margin by which you succeed or fail doesn't particularly matter. You have nothing to lose by rerolling a fail, and (except for the slim chance of a critical hit) it's not worth the risk of rerolling a success.
In this situation, at the most basic level, advantage and rerolling are functionally the same: you succeed if at least one of two dice roll a success. We can model this scenario using logic gates:
In either case, you only fail on two failed rolls, and all other possible results are a success. The two are functionally equivalent for simple cases of boolean success or failure.
However, there are a few complications, in practice:
Advantage is generally slightly better since its only drawback is that it can't stack with advantage, and lots of things already give advantage. Conversely, and to answer your question directly, a reroll's main benefit is that it stacks with advantage, and lots of things give advantage.