There are number of things. One is that with an adequate prior forecast, people can take personal actions. They can get things out of their basements. They can evacuate the area. They can secure their businesses. They can protect their farms. There's lots they can do. Lots of sandbagging can be done with a few days' notice. Provincial agencies can reduce the water storage in reservoirs to provide capacity for water to fill in.
Furthermore, better predictions can allow for better flood plain mapping and help the insurance industry come up with reliable and suitable costs for flood insurance across the country that are based on the best science. It would also allow communities, provinces and territories to plan for future development to avoid excess costs in the future.
These things can be used for water supply forecasting. It can help us look at irrigation expansion. It can help us restore natural functions to rivers, wetlands, deltas, and on and on. It's many things. Hydroelectricity is also a massive one.