Because Composites modelled with CAF (Composite Application Framework) should be built on a reliable development infrastructure.
The NWDI (NetWeaver Development Infrastructure)/JDI (Java Development Infrastructure) provides crucial development tools, e.g.: transporting DCs, using APIs, distributed development, version controlling, and many more.
Sure, it’s more convenient just to install CAF (and execute the developing locally) compared to install two components (CAF and NWDI). But: at some point you’ll be faced the big issue to transport your local developments into a development infrastructure. This reveals additional, unnecessary effort. It would be much more better to install and configure CAF AND NWDI BEFORE you start any Composition developments. This will save time and money.
Follow the link above and you’re leaded to a cookbook that helps you to execute this installation task fast and effectivly. It explains the installation of all needed components for developing Composites with the help of the Composite Application Framework. These components are WebAS, CAF (for sure), NWDI, Portal, KM and TREX. Additionally you’ll find in this cookbook descriptions regarding the configuration for the development of Composites with CAF.