Nathan Ryan, writing for The Industry:
If you’re working from the start date on an open-ended project, it’s much easier for the client to dictate terms which almost always means you end up doing more work than you signed up for when the project started.
Focusing you and your client on an agreed-upon ship date for their project means you can more clearly dictate what can and cannot happen in that timeframe because you’re working against the clock and you can only get so much quality work done in any given amount of time.
I can’t say I fully embrace Nathan’s stance; something about basing things off a hard deadline (which often changes) does feel very “waterfall methodology” to me. Yet he makes some reasonable points, especially the one quoted above.