Time-to-market and delivery on-time at all times are key values and principles in DSDM.
Turning the planning paradigm upside down
Today we are often used to delivering all requirements and adding people or calendar time when things go slightly wrong.
In DSDM we will abide by time at all times. This means being ready to throw less important functionality out of the project.
The reasons for time-boxing
Always having the possibility of moving deadlines creates dangerous mind-sets in projects and organisations.
First on the market with new functionality is often the one to make the most money.
Projects are often interdependent. A one week late project often delays several other projects and personnel at least one week too. Each week a 5-person project can thus cost hundreds of thousands in any currency you choose.
Constant re-scheduling of events in projects is very costly in itself and creates disturbances for other parts of the organisation causing further cost there too.
In effect - cutting out functionality is simply much less expensive!