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:
- The truth table for Advantage is an or gate: You succeed as long as at least one of the rolls is a success.
- The truth table for Reroll is also effectively the same as an or gate, except that the results True/True and True/False are invalid and collapse down to True. If the first roll succeeds, the result is a success, and there is no need to use the reroll, so the second roll is irrelevant. If the first roll fails, and the reroll succeeds, then you pass.
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:
- The benefit of a reroll-based ability is that you may have both a reroll and another source of advantage, which is better than just advantage; whereas you cannot stack two sources of advantage.
- The benefit of Advantage is that it will cancel disadvantage, and certain abilities may trigger on advantage, such as a rogue's sneak attack.
- In some cases, such as those monster attacks which have an even worse effect if you fail by 5 or more, the reroll can actually make things worse. You'd much rather have advantage in this scenario.
- Critical hits will happen slightly more often with advantage than with rerolls, since you'll rarely reroll an attack which hits.
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.