Bootable yes, if the USB key is big enough
That's cool. Good to know. Thanks.
It's important that it is bootable for trouble shooting purposes.
It also extremely handy that if your boot drive starts failing or fails, that you can boot off of the external until you have time to replace your boot drive.
It also makes cloning to a blank drive with no OS a piece of cake.
Not too many average users know about bootable externals or the benefits. I think it's something that is more common in the Mac community, because we've been able to boot from external firewire drives for decades. That changed when Macs started using Intel chipsets. Since then we are able to boot off of any device large enough to hold an OS.