I believe (with zero supporting evidence) that Blizzard implemented level scaling reluctantly, in order to better facilitate ad-hoc group play. Unfortunately, they kind of screwed it up.

When you move around in the overworld, you sometimes run into another player. They're fighting some enemies, you jump in to help (or vice versa) and it's amazing. Those are some of my favourite moments in the game, and the only reason it works is because of the level scaling. Even if my character is only level 10 and theirs is level 40, we can fight the same enemies and have roughly equal impact, because for me those enemies are level 10, and for them those same enemies are level 40! It's really clever, and I think they felt the sense of progression was an necessary sacrifice to enable that kind of improvised cooperative play.

"But ordinary, you said they screwed it up!" Yeah, they did. Because what happens when the enemies are dead? You continue towards your quest, and they continue towards theirs, and poof, you're all alone again. These brief moments are tantalizingly close to true pick-up experience: you start playing, meet a few people, team up, and have a blast together for an hour or so, just like you could in Diablo 2.

Oh, and even if you do happen to have the same quest, unless you took the relatively scary step of formally inviting them to your (1-person) party, the moment you enter a dungeon, you each get your own instance, and you're torn apart.

And finally, there's no global chat, so the only real way you have of communicating with people you meet prior to inviting them to a party is a Hearthstone-style emote wheel. There are at least 3 quests that require you to use the emote wheel, so they really wanted you to know it's there and to learn to use it, but in practice no one does and it's useless.

Taken together, it just barely doesn't work and it's really unfortunate. And counterintuitively those brief moments of comradery make the game feel more lonely than if you never met anyone at all. Because time and again, you're confronted with the fact that people are out there! Having fun, kicking ass, taking names. Just... you know, not with you.

The only thing I can't quite figure out is why they didn't attempt to 'matchmake' players of similar levels together. There are literally millions of people playing Diablo 4 at any one time, surely there's someone who's doing the same quest at about the same level as me? Why don't I meet those people? Or maybe the odds just don't work out, even at that scale.