hawkeyefan
Legend
I have two initial thoughts, which echo some of what others have said.
I find that maps help provide context for players to better understand the fictional world their characters inhabit. This city is beyond that river, this blacksmith is four streets from the tavern, this district of the city is going through civil unrest, and so on. I think these kinds of references, whether detailed ahead of time or during play, give the GM details to help portray the situation, and give the players some sense of what their characters can expect (so the GM can use the river as an obstacle or as inspiration for other obstacles, and the players know the characters will need to cross a river to get to the city, and so on). So the geography offers inspiration for scene framing or random encounter lists and the like.
The other, which I think may relate to the idea of linearity mentioned in the OP, is that geography can serve as a pace setter or a way to sequence elements if desired. Think of a hexcrawl type of game, with the characters at some kind of home base and a large frontier before them to explore and interact with as they'd like. The Gm can use that structure to arrange for encounters/situations that are appropriate to the level/power of the characters. So suitably low level threats may be in the immediate vicinity of the home base, and high level threats much further away. This helps pace things according to the power level of the characters, or to potentially "nest" a chain of encounters (the goblins right near base are working for the necromancer in the swamp, who serves the lich in the far off mountains, etc.). You can use it to help create decision points (do we go over the mountains, or through the mines) or to sequence certain events/locations (to get to the wizard's tower, we must brave the river and go through the harpy infested fjords).
I find that maps help provide context for players to better understand the fictional world their characters inhabit. This city is beyond that river, this blacksmith is four streets from the tavern, this district of the city is going through civil unrest, and so on. I think these kinds of references, whether detailed ahead of time or during play, give the GM details to help portray the situation, and give the players some sense of what their characters can expect (so the GM can use the river as an obstacle or as inspiration for other obstacles, and the players know the characters will need to cross a river to get to the city, and so on). So the geography offers inspiration for scene framing or random encounter lists and the like.
The other, which I think may relate to the idea of linearity mentioned in the OP, is that geography can serve as a pace setter or a way to sequence elements if desired. Think of a hexcrawl type of game, with the characters at some kind of home base and a large frontier before them to explore and interact with as they'd like. The Gm can use that structure to arrange for encounters/situations that are appropriate to the level/power of the characters. So suitably low level threats may be in the immediate vicinity of the home base, and high level threats much further away. This helps pace things according to the power level of the characters, or to potentially "nest" a chain of encounters (the goblins right near base are working for the necromancer in the swamp, who serves the lich in the far off mountains, etc.). You can use it to help create decision points (do we go over the mountains, or through the mines) or to sequence certain events/locations (to get to the wizard's tower, we must brave the river and go through the harpy infested fjords).