why use yosemite bootable usb installer when you can use recovery HD for clean install?
A bootable drive saves the entire OS X installer locally. If your current reinstall were happening from a bootable drive, the "downloading Yosemite" step would be nonexistent because everything's already there.

As in, you're wrong. The specific benefit of a bootable drive is the ability to reinstall OS X without needing to download it on the spot. (Of course, first you needed to have downloaded the OS X installer and run through the bootable drive-making procedure at some point beforehand, but the point is that once you've done that one download, you've got it for whenever you need it.)