You have a very big project to make and you know exactly what it needs to contain, examples:

1. you have an application that users already use but you need to move off a platform by a particular time or you are screwed. So you know exactly what you need to replicate in the new platform (your current functionality) and you have a set deadline to do it in. For me it was the company got sold and we needed to be off of Thomson Reuters platform within a year or pay over $100,000 a month to use it.

2. You have entered into a legally binding contract and you know what the application is going to need to contain and what you can drop without getting sued or not getting paid.

If it is your own application you can always stretch timelines out a bit to get to where you need to be, or you can decide to drop features to get to where you want to be. But once there are other people in there you might not be able to stretch timelines of drop features, and then you need something that gives you a way of alerting you beforehand if you're going off the rails.