I think fassssst means controls common across applications in how, e.g., in macOS and Windows many applications use the same controls for labels, buttons, checkboxes, radiobuttons, listboxes, editor boxes, etc with a consistent look and feel/behavior (something at especially on macOS people do appreciate).

It's not just that. The OS adds features into the native widgets. Spell checking, translation, IME support, definition lookup, reconversion (CJK). You get them all for free and the OS can add more with every version, the least being password insertion. Render your own UI and that can't happen. That's one reason why Flash died. It meant that a new platform (mobile) couldn't give the user new ways it interact where as HTML could because it was up to the browser/OS to provide the low-level widgets, not the app itself.