If you notice, the final battle takes place in what appears to be a monsoon: torrents of rain coming down in the middle the fight, adding mud to the equation and generally making life difficult for a bunch of people who just weren't miserable enough.
From a visual sense, it makes the final battle really dynamic and exciting, providing a fun-filled X-factor in a chaotic battle where people are dropping like flies. But from a thematic standpoint, it makes a very powerful statement about the nature of the world and how this conflict plays out beneath it. In the west, we're used to seeing justice done at the end of our movies. Good guys win, bad guys lose and that's just the way it is.
But Seven Samurai carries much more moral ambiguity, painting victory as a fleeting thing and defeat inexorably intertwined within it. In other words, the universe just doesn't care one way of the other, and if they bad guys get away with it, then it won't do anything to stop them. The universe is arbitrary. It rains on the righteous and unrighteous alike. And while you can count on it to not bail you out under any circumstance, at least its arbitrariness is somewhat predictable.
And again, this echoes what Japan was dealing with at the time the movie came out, and how they learned that things don't always turn out the way you want no matter how hard you try. If it's gonna rain, it's gonna rain, Kurosawa tells them. They can respond like the samurai do and get the job done regardless, or they can let its fundamental lack of justice overwhelm them. That's a bleak lesson, but the times were just right for it. Seven Samurai knew how to deliver it in a distinctly memorable way.