Sometimes you need a valid-looking email address as a placeholder, but you need to be sure that it truly doesn't exist — and that it's unique across your whole database.
For example, a lead's Email Address can be blank or have a basic syntax error (username missing @domain, only a domain with no username, etc.) in Marketo itself, but validation errors can stop the lead from syncing to your CRM. Not so good, since those are the very leads Sales needs to track down using other info.
But what value can you fill in that simultaneously…
[a] is unique,
[b] is correctly structured, yet
[c] is fake-looking to the naked eye, and
[d] per technical rules, can't ever be a real-world mailbox?