Looking purely at DR indeed there should be little reason to use something on top of D810.
What I cannot explain (due to my lack of knowledge) is that when I shoot, e.g. yellow flowers with my D810 vs the S5 (using an ETTR approach) I find that I can recover much more to my liking a nice saturated yellow with the S5 vs the D810.
Not sure if it has to do with the way channels are saturated or a film like shoulder on the S5 or something else.
On the other hand it could simply be me trying to rationalize why I keep another camera around