In my opinion, the placement of powerful enemies (stronger than you, at least) in accessible locations creates a funnel in which you have to do a bit of your own Pathfinding to find a safe route.
In a world so open, movement and accessibility is channeled through the flow of landforms ("This mountain is too tall to climb so I need to cross this bridge and go through this cave", etc). However, the enemies create an artificial, almost psychological barrier ("I can't cross this bridge, its guarded by level 50's!").
While these areas are easily accessible physically, the enemies provide a sort of "at your own risk" level of danger. What's important is that as you become more powerful, you can overcome these enemies and they stop becoming a roadblock and the world begins to open up more and more.
It's a big world, and so in terms of efficiency, the ideal path would be a very "point A to point B" approach. I feel like the added level of danger simply allows for more high stakes "exploring" and if you skirt around tough baddies, you may well discover something you wouldn't have otherwise.