I gave up trying to explain this to people when I was bitching about the exploration in Xenoblade. All there was to find in that empty world was (more) EXP for discovering landmarks, bosses you couldn't kill until later on with the shitty battle system, and minerals that were only useful later in the game when they actually had more of an effect.
In Dishonored, I think the paths are functioning with the intention to cater to completely different builds and playstyles. Because of this, they're not necessarily something that shines brightest on a single playthrough. Assuming you only play once, their main use would probably just be one's angle of approach. Beyond that, they provide the opportunity to back out of a path and seek another if you find yourself ill-prepared to continue down the one you were.
Your gripes about the exploration in the game are fine. You won't need to explore if you're content with your runes, bone charms, money, and elixirs. It's not like a Metroidvania or some dungeon crawler where poking around will often reward you with something new or interesting.