IME, because client asked for the different one for whatever reason and is willing to pay for higher development cost. You can argue about that just the same a

IME, because client asked for the different one for whatever reason and is willing to pay for higher development cost. You can argue about that just the same as when client needs a feature X. In my case, I was very often getting away with YAGNI or 'how about we implement nightly sync from pg to oracle' but not always.