We all know discs are inconsistent, if you only ever have one [driver A] and you lose it and get a new [driver A], the chances of it flying the same are pretty low, especially if you had the first one for a while. So if you pack 2 or 3 of [driver A] in your bag with some variation of weights and plastics then you get a better feel for the mold overall, and you get more shots covered by discs that feel the same in your hand.
I throw two Lat64 drivers, and having them in Opto and GL gets me predictable variations in stabilities. I know my GL Saint has more turn and if I keep it low, nearly no fade, so I can pump it low and hard on tunnel shots, but my Opto only turns a little and fades reliably, so for safe hyzers or shots in the open when I wanna put more mustard on it, I'll use that one. It's a bit lighter at 164g, so if it's windy I have a heavy opto that will turn but fight back in a moderate wind, or fly more overstable in calm conditions.
Or, for Firebirds, they're both Champion, but I have a domey one and a flat one. The domey one is my strong wind driver, dogleg disc, etc. The flat one is purely for utility, it's so overstable that it's all but useless to me on a BH air shot, even in a strong wind.