A traditional waterfall project releases no value to the customer (i.e. they have nothing to use ) until the final, whole project is delivered. This means the product cannot start paying for itself, cannot start generating revenue or even make a profit until the whole of the project has completed.
Even the newer monolithic methods such as RUP, fail to deliver their true value until the end of the lifecycle. Why else is there a transition phase in RUP, its only really at this point is the product in a suitable state to go live and begin paying for itself.
If a project can be split into many small segments, each of which produces an output which can potentially go live, it can start realising ( i.e earning ) a percentage of the overall value of the project. This release of value can be built upon after ever successive release, meaning the project can pay for itself in a much smaller timeframe and start earning tangible profit from a much earlier date than with traditional delivery methods.