I'm quoting OP from another topic to bring the discussion here:
When you 'generate' a Note on the website the private key you are getting is a normal Bitcoin private key. It starts with 'ww-' so it's easier to distinguish from other bitcoin private keys you have in case you save them in the same place.
The Note Public Address is in fact the Legacy Bitcoin address corresponding to the private key you saved, the only difference is that every '1' is changed to 'ww' so again, you can easily distinguish a Whirlwind Note from a Bitcoin address.
Example:
ww-L4xK361wZYYxJg7vSwXgDTVBVXY4JfgYrUU5QZic2nNB9PUbMzbt - Note Private Key
ww8Sf8x3GBUiTxg55RQEzynf2Cdyy7F1ihh - Note Public Address
L4xK361wZYYxJg7vSwXgDTVBVXY4JfgYrUU5QZic2nNB9PUbMzbt - Bitcoin Private Key
18Sf8x3GBUiTxg55RQEzynf2Cdyy7F1ihh - Legacy Bitcoin address
If you do not want to generate the Note on the website you can simply use any other private key and it will work the same.
Example: Imagine you need to receive a payment so you generate a new address locally and send the Legacy address to the sender expecting a normal Bitcoin transfer. The sender can now pay you instantly, anonymously and for free through Whirlwind even if you didn't know we existed. You could then access the website and withdraw your funds to your desired address. (the sender can also send you the LoG for the Pay to Note transfer proving that he sent the funds)
(I've shortened the quotes a bit to focus on the relevant parts)
I have some doubts: if I'm expecting a Bitcoin transaction, I wouldn't appreciate being told to use a third party to collect my money. The sender could instead have withdrawn the note himself, and sent an on-chain transaction to my address.
As a sender, I also wouldn't really want to rely on a third party to send funds and provide evidence. No matter how trusted your service becomes, it's never as strong as on-chain evidence. Unless you don't want an on-chain transaction trail of course.