"You can't just turn on creativity like a faucet. You have to be in the right mood…last-minute panic!" Calvin and Hobbes, Bill Waterson
Constraints have a great way of bringing focus to work. They have also been shown to have an effect on creativity. Having unlimited options and possibilities, in fact, makes it harder for us to make decisions (decision fatigue). When we create a product plan, we are dealing with a number of options and have many decisions to take. These can range from something as high level as what feature to build, to something as finely detailed as where to place the purchase button.
When faced with so many decisions, and many of them being in the speculative zone, setting up a constraint can help to bring focus. Time is a clear, tangible constraint that has an intense effect on our psyche. Time lost cannot be recovered, but our tendency is to keep extending time. If we're not ready, then...