We had a 850 gallon tank connected to our rain gutters which we could run into our well when it went dry. That worked really well and was more than enough to get us through the dry season.

If you have any plants in containers, rainwater is good for flushing out the salts that have built up.

What's the soil like? If it's fast draining your neighbors will love you.

OP, what sunset zone are you in?

This is why I haven't seen the point where I live. I am in N Cal, central valley climate. I am told what makes sense is a huge underground (under the lawn?) holding tank, which can hold enough to to be there when you need it. Not sure what that would mean in gallons.

Sunset 24. Sometimes we will get a 1/2" in the barrels just from the morning dew (marine layer).