Some Mac users have more than one browser. Some of us have half a dozen browsers. If you struggle to decide which browser to use for any given link, Choosy adds another choice to your browser confusion.
Forget the default browser, Choosy opens links in the right browser. When you click on a link Choosy will do the right thing, whether that’s something simple (like using whatever browser is already running) or something complex (like prompting you to pick a browser…).
I struggle to imagine that many Mac users have the problem which Choosy solves.