Not all projects need a formal plan but plenty do and even those that don't officially need a plan can be hard to visualise with out a properly prepared drawing.  Our experience has shown that the customer's original design is not always interpreted correctly by the person who actually has to build it which is why we offer a design and build package so that we can be part of the process at the beginning.  Of course using us also means you don't have to search for a reputable architect as well as a builder.