The worst part is with quest markers, is that the game isn't built with those who don't use them in mind.
In games before quest markers, you were given detailed instructions on how to get to places, but nowadays if you don't want games to hold your hand, tough, as there is no way to find your way round without them markers.
Quest markers exist because directions really aren't needed anymore.
To use Bethesda's games as an example, since Bethesda basically popularized the use of quest markers. Morrowind had directions because the in-game map didn't have the detail needed to mark every single location on it. It could only mark major towns, and a few of the larger MQ related dungeons. Since the map couldn't display every place in the game, they needed NPC to give out directions to places, or else you simply wouldn't have been able to find it.
The problem with directions stems from the fact that everyone's ideas on what constitutes something are different. If the directions are, go east until you reach the big hill, then turn north until you reach the gnarled tree, or w/e, people can be left with the question "what defines a big hill?" and "how much does a tree have to be messed up to be considered gnarled?" which only leads to people getting lost.
With the advancement of graphics, and thus, in-game maps, theres no need to give out directions anymore because maps are detailed enough to display every location, and now devs can actually have the maps be something other then a "mechanic" and start being something actually recognized by the game as a thing your character has.
Why would someone give you directions to something/place when you can just pull out your map, and have them show you where it is? Especially in Fallout where your pipboy as satellite maps on it. The markers exist as your character remembering where they were told to go for a particular quest.
Not only is it more logical for people to show you where places are that way, but it also saves on VA costs. So its a win/win there.
Though I do think some game devs abuse it heavily, with some scripted linear sequences having a marker above a guy you are supposed to chase after or w/e, but hes basically the only person in the screen, so its impossible to ever really lose him, making the marker pointless.
I also think map markers should end when you reach the front door of the "dungeon" or w/e. We all know the thing is always in the boss chest at the end, as per RPG cliche, we don't need a marker telling us that.