Disposable by design
"Disposable" is the whole point: the address is meant to be abandoned. Because it is thrown away, a leak of the site's mailing list costs you nothing, and there is no unsubscribe process to chase months later.
How this differs from a spam folder
A spam folder still ties unwanted mail to your identity — the sender has your permanent address, and filtering only changes where the message lands. A disposable address means the sender never had a durable way to reach you at all.
What happens to messages
Incoming mail is parsed, HTML is sanitised, remote images are blocked, and the result is stored against your inbox for the length of the countdown. When it ends, messages and attachments are deleted rather than archived. See the privacy page for specifics.