There are times when I don't prepare for games but I let the players drive everything about their characters. You know this kind of game. The GM just starts the session with, "So, what's your character up to? The last time we saw them they were..."
This is great in a group of improvisers. Everyone just responds to what the others are doing in that moment. I play in a game of PTA that does this every single time and we all have fun with it.
On the flipside, there are games in which I plan much more. I start the session with a recap of the previous events and often set the first scene. To raise the bar more, I might have several scenes prepared to open throughout the game. This works well when I'm playing with my kids. They're quite young and they like a linear story so that's what I usually give them.
Another way to think about that kind of preparation is what I gleaned from Spirit of the Century a few years ago. It's not about scene preparation. It's about antagonist preparation. Their example revolved around the question of what the antagonist would do in the circumstances of the game. That is, they wanted to steal the sacred jade egg from the museum. If they were foiled by the player characters, what do they do now? Do they try and steal it from the player characters? Perhaps they weren't foiled by the player characters. Now we need to know what they wanted to jade egg for (some ritual?) and then add that.
I think of this as a kind of intervention encounter. GM characters need to have their own motivations and plans, with contingencies, or with new plans that they formulate only because the player characters meddled in Plan A. These kinds of interactions, when committed by a meaningful GM character, connect the character to the larger story. All the other encounters run the risk of having no connection to the details on the character sheet. A character might be described as the second son of the village chief, but the encounters might always be about defending against coastal raiders. The disconnection is a wasted opportunity.
And round out this post, I also use a fourth kind of encounter, the random encounter. We know they add a little spice to a story, often by giving a chance for players to show off their character in some way. It's a good pipe beat that can return later in the story climax. The other thing I like about random encounters is that the universe is random, and sometimes it makes sense that life is also random. No one expected that storm, that car accident, that economic downturn, or whatever.
And yes, I have a game to prepare for tonight. Better get on with it.