The benefits of random encounters are that it creates a sense of danger. At any time, you could be attacked. You never know, even if you've played the game a hundred times and even if you've cleared out every area from all planned encounters. It creates the illusion of a living world, not static where once you've cleared it, nothing lives there anymore. It's just a ghost town. It creates variety and spices up the game so you never know what you might face this playthrough.
