It’s an advertisement by Yubikey - the hardware key manufacturer
The first "hardware key" is actually my Bitwarden faking a hardware key (I'm sure they'll start blocking BW because of this in the future) but it doesn't let me add a second one unfortunately.
If you’re using real yubikeys, it’s protection against losing one. If you had two from the start, you’re not at risk of losing your only way into your account when one goes missing or is stolen.