If a party were to have it's way, this is the formation it would be in no matter where, or how far it travelled:
And that's very understandable. The moment something jumps up from the bushes, its best if the fighter is right there and if the mage is far away and out of danger. It's even better if everyone can take an attack on the first round, killing it quickly.
However ... people do, occasionally, have to go to the bathroom. People get distracted as they journey along a road, they lose sight of the potential danger when mile after mile has to be trekked. Petty disputes arise between people, with sore feelings and a strong desire not to walk within fifty feet of each other. People have to stop and retie their packs, or sort through for some food. Mounts get stubborn and upset, and burrs have to be pulled out from under their saddles. There are a hundred ways in which a party gets split apart - in the real world, that is.
Take a hike with five or ten other people sometime, in the Rocky Mountains say (which are right by me). Warn them about bears and cougars, about watching their footing and about not wandering off the path ... and then set out through some dense bush for ten miles. See how long it takes them to drift apart.
I remember being nine, and being on just such a hike with one hundred other boys, as we walked down from the cub camp where we'd been, to the parking lot where the cars were. The whole walk couldn't have been more than three miles. But I can remember at one point on that route being all by myself, other kids behind me and other kids ahead, but too far away to even hear them. I remember relishing that walk, all by myself amid the mountains and the sounds of the forest.
But suggest to a party that they're too far apart to hear one another as they strike out in the wilderness, and they will soundly protest. I've had players protest against their characters going off to be alone behind a bush so they could pee. "I'm not shy," they'll say. "Manners were less important," they'll say. "Nobody cared about people shitting in front of them," they'll say.