I've read about GOG's regional pricing changes and the games recently removed from the catalog. I think I understand most of the developments. But somewhere along the way I missed the reason they switched to charging local currencies. Is it a necessary component of regional pricing, or something unrelated to that?

First of all, I want apologize sincerely for the useless answer, but I can't stop myself from making a pop reference.

"It's for local people only." -- The League of Gentlemen

It's very useful, there has always been a constant stream of complaints that because of the dollar only billing: many users have faced high fees every time they buy, around $2-$5 transaction fee + 3%-5% on the currency exchange, some banks and credit cards in some countries don't allow foreign transactions at all, and some users are relentlessly plagued by anti fraud systems blocking transactions that suspicious due to being low value foreign transactions, sometimes several in quick succession. All a royal PITA, which billing in home currency should improve a lot for those users affected.

^As IanM said, plus you can choose the currency you want to use :)
I don't have foreign fees, so I'll stick to USD for the moment.

(Just adding to what IanM said) ...At the same time, GOG gives the chance to whoever wants to keep paying in US dollars, to do so.