Doing project work generally yields any of three results: either you complete the work in good time, you overrun, or more work comes out of it. They are not all mutually exclusive. The problem with projects is that work tends not to be done in good time. Usually work overruns and/or generates more work.
Suppose we’ve got two pieces of work to do. An important item and a less important item; we expect to do them both in the time available, and they both “must” be done. However, circumstances prevent us doing the most important item first. Never mind, “it’s all got to be done”, we think, “we’ll do the less important work first.”
But now there is a danger that the less important work will overrun and reduce time for the more important work. Or perhaps more important work will take longer than expected. It could well overrun and/or generate more work. As we approach the deadline we find delivering everything we wanted is no longer achievable.
If we had been able to do the most important work first then we’d have discovered these problems sooner and been able to do something about it sooner. Perhaps we would have chosen (reluctantly) to jettison the less important “must-have” work. We’d have delivered less than we would have liked, but at least we’d have delivered the essentials.
These observations are partly directed to people who do the work, but also to those who manage the environment that people work in. If your team cannot do the most important work first—perhaps due to lack of access to a particular person or building or technology—then prepare for them to deliver late.