California's snow pack is essentially another "reservoir" that is able to store water in the Sierra Nevada mountains. Graphing these things together can give a better picture of the state of California's water and drought.
Updates daily and can be fun to watch while the storms roll through
Rainfall is measured in inches. If you want to grow a specific crop, you can look up how many inches water that crop needs over a growing season. Rainfall naturally already includes an area factor in it, but if you're spreading a volume of water over multiple fields, expressing that volume in acres*inches or acres*ft is relatively reasonable.
It's also a reasonably direct way to measure the amount of water stored in a reservoir. If you know the area of the reservoir in acres, then any height changes can just be multiplied by the surface area of that resevoir.
I'm all for metric in science and manufacturing, but this particular odd unit does seem to fit the application well.