This is rather like asking, why do I need a high-level language? The strict answer is that you can make do without tools to help you build applications, but you will be making your task an order of magnitude harder.  If you decide not to use 3L Diamond to build a multiprocessor application you will have to do all the work yourself, including things like:

• loading all the processors, including ones remote from the host PC
• keeping control of all the separate modules needed to load your application
• starting your application in a synchronised way
• managing communications, possibly including deadlock-free message routing
• writing your own device drivers
• explicitly managing all memory allocation
• writing your own multithreading support or using something that is likely to be less efficient than Diamond's
• inventing a host communication mechanism
• being prepared to rewrite your source if the configuration changes
• being prepared to make major changes if the underlying hardware changes
• supporting multiple source versions for all hardware and configuration variations
• handling all of the underlying hardware peculiarities (unexpected cache behaviour, for example)
• ... and many more.

Diamond can do all of this for you. 3L works closely with hardware vendors and puts a great deal of time, effort, and experience into optimising all aspects of the system to give you the best results.

See the more complete article here.