Many non-Japanese conflate sashimi and sushi; the two dishes are actually distinct and separate. Sushi refers to any dish made with vinegared rice, and while raw fish is one traditional sushi ingredient, many sushi dishes contain seafood that has been cooked, while others have no seafood at all.
It's the moisture and contaminents that break down the DOT 3 and 4 fluids which is the reason to change the fluid in the first place. If you don't change it, what happens is moisture enters the system via the vented fill caps on the brake master cylinder. This moisture along with dust, mixes and travels in the system eventually making it's way to the lowest points in the system.
Your car's brake system works on a hydraulic pressure system. Brake fluid serves as the hydraulic fluid that maintains that pressure. A leak in brake fluid will cause a drop in pressure, possibly resulting in brake failure. That's not something you want to happen as you're cruising down Dead Man's Hill going 60 MPH. Fortunately, in most modern cars, brake fluid leaks are rare.