Aaron Franklin puts a water pan in his offset for brisket. A moist cooking environment leads to better bark and smoke ring.
Whether you need the water pan is highly debatable/debated. I see your Aaron Franklin. And I raise you a Harry Soo.
Harry cooks in a WSM with charcoal. Aaron does uses an offset stick burner. Since we're cooking here with a WSM and charcoal, I think it is good to consider what Harry says too.
Harry uses an empty WSM water pan for brisket. Aaron uses a water pan in his offset. Harry uses a slather for brisket (which adds mositure). Aaron doesn't slather.
Both Harry and Aaron spritz (which adds moisture too). Harry also injects brisket (which adds moisture).
My $0.02 is that the filled water pan in a WSM is primarily about temp control, and not really much about the moist cooking environment.
Smoke adheres best to the surface of cold, moist meat. And the smoke flavor is really just on the meat surface -- it doesn't penetrate hardly at all. The brisket (with or without a water pan) is cold going going in. The brisket also sweats out half of its starting weight over the course of the cook. After all, the sweating and evaporative cooling is the whole reason why the brisket stalls for many hours (unless wrapped). So if you also add moisture in the form of slather and spritzing and injection, hard to imagine that you need even more moisture coming from the water pan.
I think you should use a full water pan at first to get a brisket baseline. That's what I did.
But over time, I've migrated to Harry's method -- dry pan (actually a FireDial), slather and spritzes (but no injection so far). Seems like plenty of moisture for fine bark. Since I'm OK controlling temps without water, I'll take the better fuel consumption, less fussing (no pan refills; fewer charcoal refills), and easy clean up.
YMMV.