why use yosemite bootable usb installer when you can use recovery HD for clean install? i just re-installed Yosemite from Recovery HD. Performed a clean install as i erased my HD from Recovery HD before installing the software.

Yosemite is being downloaded since a few hours but it would be the same with the USB Installer as Recovery HD is an Installer too.

So in both cases you need internet.
Am i right or wrong?
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.)