When we start talking about anything and everything to do with project management, it's best to start with the end in mind. What is this project supposed to provide for the customer? What Capability will the customer posses when the project is complete? This is the Capabilities Based Planning paradigm used in our Defense and Space domain. CBP started in the Australian Forces, moved to the Canadian Forces, and came to the US DOD. From there it is a natural jump to any project based planning process to ask what do we need to successfully complete our business mission?
So here's the framework for starting with the end in mind, as Mr. Covey advised years ago. The Capabilities being delivered on or near the need date, at or near to desired cost are developed with the following steps. This notion of on or near is critical. If the Capabilities show up later than needed or cost more than planned, their value to the business is diluted. By how much? Depends on the amount. There are many examples of projects way over budget, way late that still delivered value, Sydney Opera House, DIA here in Denver. But another example of if the date was missed by 2 weeks on a 5 year program with the result of total loss of mission.