Let's say I'm running a game and you are playing a character who has a Sword of Giantslaying and a Ring of Fire Resistance. Cool.
When are you going to run into fire giants?
1
In a game focused primarily on your character and their adventure, you encounter fire giants when I decide to introduce them. When I want to see your character kick ass and do their thing, I bring the fire giants. But when I want to give you a challenge, I bring the frost gnomes. When the frost gnomes show up, you can't just fall back on your fancy sword and your magic ring, you have to rely on your other qualities.
Or I can give you a choice. I can put the threat of the fire giants over here, and the threat of the frost gnomes over there, and let you decide what order you confront them in. But the point of including them is still based on their relation to your character: the fire giants are a threat your particular character is uniquely well-equipped to deal with, and the frost gnomes are a threat that forces you to come up with a different solution.
This ends up being a deliberately narrative process, driven by our understanding of what an interesting story is. Instead of relying on randomness or chance, it relies on the sensibilities of the people playing the game. Thus, it tends to feel more like a coherent narrative arc.
Or, perhaps it feels too staged. Like every situation has been crafted for you. You might feel too much like a character in a story, instead of a character in a world.
2
In a game focused primarily on exploring a map that one player keeps secret from the others, you encounter the fire giants when the map tells you to. Either the random encounter tables say they show up, or you enter the area of the map that says it contains fire giants.
Ideally, the map is created either through random procedures, or before the players make their characters. This tends to make it feel more “real” because it relies on players exploring an already-fixed structure instead of relying on the narrative sensibilities of the participants. When you know the encounter with fire giants is random, instead of planned, it can feel like you lucked out and hit the jackpot.
Or, perhaps it feels too random. Like your decisions don't matter and none of the things you do have any connection to each other. You might feel lost in a world that has a life of its own but no connection to your character.
3
To make this difference even more abstract let me put it this way:
A good story is a series of events that are somehow connected, arranged in an order displaying cause and effect. Anything not related to the specific events the storyteller wants to highlight is abstracted out. Thus: a series of situations arranged in time.
A good map is a collection of locations that are physically connected to each other, where interesting things happen (otherwise you wouldn't want to go there). The map has borders, leaving off places that are not important to the specific locations the cartographer wishes to focus attention on. Also, places on the map might be abstracted as well, leaving out small details that are of no consequence. Thus: a series of situations arranged in space.
Also it sucks to have a Sword of Giantslaying and a Ring of Fire Protection in a setting that has no giants, no fire-themed monsters, and no fire-themed locations. But that's a matter of building a game with a self-contained setting.
4
Obviously this distinction is to some degree artificial for the purposes of comparison. Few actual rpg campaigns are exclusively focused on one or the other, and you will notice that this is not an important distinction being discussed by other people. I like both types of games, but I also like to know which one I'm playing, so it's important to me, at least.
Call of Cthulhu uses a relationship map of clues instead of a spatial map usually. Yeah fuck it, send tweet.