If the direction they choose to walk doesn't matter in terms of which encounters they're going to face at which point, then - to my way of thinking - the map doesn't matter, because it's really just Schrodinger's Railroad. I recall several discussions here and on the old boards that indicated this style of DMing where All Roads Lead To Rome is "lazy" or "bad," and would prefer to avoid it as a solution.
Main factor in favor of the technique is time. You have limited time to create detail, content, conflict, growth and progression. Without the method, you can easily wind up with upwards of half your created content never being relevant, you just wasted the time and effort that could have gone towards making better quality conflicts which the players DID encounter. Heck, locations don't really 'exist' until players interact with them, or with their knock on effects.
Thats the difference between GMs and module designers. Designers must aim for completeness, but not detail, you must have content for every plausible place the PCs visit within the bounds of the adventure, and advice for the implausible ones. GMs must improvise and act to keep everything going. Matters are mutable in the face of that.
This is not to say the extreme of having every route be the same. You can recycle the starring conflict, but revise the secondary and environmental attributes. You can create a location map, for example, as a static anchor, while having furniture, creatures and activated effects be mutable. And even IC, they are mutable, they can be moved and repurposed for IC-wise, entirely arbitrary reasons. In fact, when making heavy use of the Schrodinger method, you very much want large static things like rooms to be predefined regardless, so you don't slip up and give the game away.
So for example, you could battle the BBEG in this throne room, private labs, bedroom or even his secret chamber. Depending on which location you encounter, you face different complications associated with the same core encounter. The throne room might have waves of mook reinforcements, the labs might have some magical creation of his to be unleashed, as well as environmental hazards to abuse, the bedroom might be strongly warded with personal defenses, and the secret chamber could be brutally trapped. Get to whichever room out of sequence and you just deal with the secondary hazard instead, with a hook included to point you to the right sequence.
Again, this is for the major points of conflict, where you need significant preparation. Random events or minor conflicts can vary, it is sufficient simply that there BE a conflict, not that it be a specific conflict.
