the correct name is "Casascius" and it is not offline storage, it is a physical bitcoin and this type is not exactly meant to be used for storage. they are designed to be used as "collectibles" for those who like having collections such as hobbyist, etc.
they also are meant to have a fixed balance (the balance that was originally put in that key) and remain that way meaning you should never spend from that key (which requires physically revealing the private key) because it makes stop being a collectible.
Well, technically they are not a "physical" bitcoin, they are just 10% of a paper wallet generated by someone else (Mike).
When I say 10% it is because the public key is not even fully displayed on the coin, just part of it, and to find your full public key you have to search for it the
database about Casascius coins.
Then, unlike a regular paper wallet, the private key is concealed, and only by unpeeling the coin, you can see it.
They are definitely used to "store" coins (private keys) as there are about 44 000 BTC stored on them.
And you can definitely spend them, as about 45 000 coins loaded on Casascius have been redeemed so far (in 2019 about 530 Casascius were peeled, it is 10 a week !).
Finally, it is not a physical BTC, as BTC never move away from the blockchain to become an object.
Casascius coins, are collectible and a way to spend / move / trade BTC off chain (same as openDime).
