It’s hard to visualize vast quantities, but for people who live in the rural American west, an acre-foot is relatively easy to grasp. People know the land area of property they live on or farm in acres, and they can imagine what it would look like if that area was flooded with a foot of water.
If a farmer knows they have a 100 acre field and they know that their corn requires 2 feet of water per year, they know that they must acquire 200 acre-feet of water.
If the state’s rivers are overflowing and the farmer wants to flood their field and help to recharge the groundwater [1], acre-feet is the unit they’d use to calculate how much water they can accept (depending on how many feet tall their levees are).
California is 100 million acres and there are 10 million acres of crops.
Acre-foot is not an obsolete unit - it is frequently used in American when measuring large amounts of water for agriculture and reservoirs. It's about 325,000 gallons.
