Wikipedia

You can design a game without open world and the players don't even notice it.

I think part of what that's about, and what's important for me, is a sense of agency. Giving the player choices, and importantly including implicit choices the game doesn't explicitly tell you about, and reacting to those choices.

I find it really lame when a game puts you through basically a linear game, and at the very end, after a convenient save point, tells you to make The Big Choice (probably That Decides The Fate Of The World), because that feels completely meaningless - as opposed to the game for example telling me to do something, some fundamental gameplay element, and at a crucial story point if you refuse to do it it doesn't fail you, it offers a different path forward.

Doing an open world feels like a conceptually simple way to give players a sense of agency and set more things up. If you see something cool, you can go there, if you can make it. You're not required to stick to the path, you're allowed to explore, look for things to do, or run straight to the big objective. And if you do run into something optional, help a character, maybe they show up in the grand finale and play a role, and it makes those encounters feel rewarding.