Probably to reduce some friction from the process. They also don't charge a transaction fee(as noted in another comment), so i could see myself using this on my site.

Of course a much useful service would be something like flattr. IIRC, There was some talk about them starting to accept bitcoins but i haven't heard about that lately.

And where do you get your Bitcoin address from? Options are: from the official client (many reasons for and against running it), one of the other hosted or local Bitcoin wallets, or Coinbase. Coinbase is an attractive option as it's extremely easy to setup and to cash out the Bitcoin directly into your bank account.

Coinbase has an easy option to convert those Bitcoins into USD on a regular bank account. This might be interesting for people not planning on keeping BTC around but who like the "close to instant" nature of it.