As you say Bryce, the wilderness as a hex crawl and therefore the journey IS the destination... I think with that in mind, you could say the purpose is "exploration". So ask "why explore?"... I'll tell you why: because there's an expectation that there is something interesting to be found.
Therefore, by the magic of transitive properties, the reason for hex crawls is because there is something worth exploring, and hex crawls are simply the best mechanical option for navigating/exploring larger areas worthy of exploration (hex maps are just compass-friendly grid maps, after all).
Now there's always something interesting to be found in an exploration game ... sites and whatnot. This is not unique to hex crawls, true. Anyone could get that type of exploration using any system. No, the real reason for exploring is because there's an expected cohesion in your world - that you, as the GM, have created a legit interesting environment that's all tied together into a visionary "big picture", and that the only way to see the "big picture" is to eat it bite by bite (or explore it hex by hex, if you're into the whole "accurate communication" thing).
If Middle Earth were described as "Moria is in the mountains, Weathertop is near Bree, the Shire is in the West", people wouldn't give a shit. It's the whole story of Middle Earth that is interesting - all the people, the history, the events - THOSE are what people want to know about, and you can't get a good picture of that by simply visiting a few major sites of interest. You've got to see the forest that Saruman cut down to build the goblin spawning pits, complete with pissed-off ents, flooded tower, and funeral pyre. You've got to see the shattered walls at Minas Tirith, with mangled orcs, dead men and elves side by side, and the splattering of Gondor's steward. You've got to visit the spot where Frodo dropped the ring into Mount Doom, beyond the black gates of Mordor, past the caves of Shelob, under the eye of Sauron.
People don't explore Isle of the Unknown because they want to see a specific interesting tree or monster cave or whatever - they do it because there is a bigger picture to understand... a story to tell... a mystery to reveal. What is this "Island of the Unknown"? How can we make it known? Explore the fuck out of it, hex by hex if need be. THAT'S why hexcrawls are a thing. THAT'S the purpose of them.
Resource management, meaningful choices, strange encounters... it's all just pieces of the greater puzzle. Things to keep the exploration moving. The real treasure was
inside us all along finally understanding the place you're exploring.