Why? For the owner: It's fun, its a way to make your coin stand out from the rest, OR use it as a wallet.
For me: I was simply curious how many coins had extra bitcoin.
How? Send any extra amount to your Casascius Bitcoin Address and it will register on the website as a coin having extra value. Example: You have a 1 BTC coin and you send 0.01 BTC to it, making the total received to be 1.01BTC. That will cause my script to flag it as Extra and it will have the + sign by it's USD value.
Seriously, make an encrypted backup of your wallet, put it on dropbox or gmail. Then, when your house burns down including computer and casascius coins, at least you'll still have your Bitcoins!