Obviously none of us are speaking with personal experience yet, but the reviews vary a bit on the difference between the Pro and Pro Max cameras. I don't think saying the difference between the two is "almost non-existent" captures every reviewer's take. MKBHD came down on this end of the spectrum, but others such as
Matthew Panzarino thought the difference was substantial:
"The iPhone 12 Pro already delivers impressive results year on year, but the iPhone 12 Pro Max leapfrogs it within the same generation."
Arun Maini
reaches a similar conclusion, and even argues that going the 12 Pro to 12 Pro Max difference is bigger than 11 Pro to 12 Pro. That seems a stretch based on other reviews, but there it is. So, I would say the consensus is more like "the Pro Max has slightly better cameras which won't be noticeable in most settings (e.g. outdoors in good light), but are significant in certain circumstances".
Beyond the cameras, there's slightly better battery life, and well, some people just plain old like big phones.