speculation present

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.

Americans will use literally any other units than metric.