EDIT: I also see Yubico Fido Keys which are $20 each (and $36 for 2). Are there any differences between these and the regular Yubico keys?
You will read lots of people talking about the cool things they do with their Y4s, but really they're just doing it because they can, not because there's a well-thought-out security benefit they're getting (I'm as guilty of this as anyone). 95% of the benefit of a security key is simply U2F.
Being able to leave it in instead of having to take it out of your wallet and plug it in every time makes a big difference in usability, which in turn makes it much more likely that you'll want to use it everywhere.
(In other words, what's better about having the security key being on your keychain vs. be a permanent part of your computer)
If someone gets your laptop with ubikey inside, they can't do anything without your password. And the whole point of a physical second factor is to basically limit the 99.9% of the people (mostly from other countries and cities) that have no physical access to you from hacking you.
I have 2 laptops and an Android device, so if I was to start using these things it would be convenient to keep my private keys available across them all.
I have the Nanos in all my computers because I'm used to that setup from working at Google, but that's more expensive than what I actually need.