First, OPML is used to exchange subscription lists between RSS aggregators. I'm the one who made this choice, because I thought it would be cool if people could edit their lists in our outliner (at UserLand) and OPML was the file format of the editor. All our competitors wanted to make it easy for our users to switch to their products, and Radio was the product that got all this RSS stuff going with users, so it became an instant standard
. And even though few people use Radio today, the choice of OPML is still with us.