in some parts of the country (US), interstate water agreements promise a certain volume of water to be delivered from one state to another, in which cases, runoff is sometimes required. Legal agreements.

Yes, it's not about capturing every drop -- more about slowing it down so a fair amount can percolate in.

The recent tragedy in Texas is a key example -- way more water than could be "handled", and if there were mitigations in place it could have been win/win (temper flooding and replenish aquifers.

I follow a youtuber who's trying to rehabilitate desert land by slowing the water down to let some percolate in -- it's a wonderful dream: https://www.youtube.com/@dustupstexas