First off, let’s state the obvious… all plants need water and water is expensive in California. That alone is enough to make avocado farmers jump for joy when a storm comes in. Rain can be worth as much as $20 to $100 an inch for every acre of orchard.
But there is an even more important reason. Avocados are sensitive to salt, and all irrigation water and all fertilizers (organic or conventional) carry some salt with them. Too much salt stresses the tree, slowing growth and reducing production. Rainwater cleanses this salt from the tree’s root zone.
The classic symptom of salt stress is “tip burn”, seen above. This is quite common on residential trees who often don’t get a deep watering or are planted in poorly drained soils. (The speckly discoloration on these leaves is from Persea mite, but that’s a different article.)