Steve Blank has done an excellent presentation for the Cleantech Accelerator conference on the difference between business plans and business models and how they can complement each other. He provides very clear examples, templates, and a structure on how diagrams can be used to clarify your business hypotheses and plans. Business clarity and simplicity have a crazy way of providing focus which can save a lot of time and money.
This applies to all types of business (and personal) endeavors especially for those in new businesses and the public sector. Some good candidates for business model diagrams are where major change is planned, there is a complex web of alternatives, and end objectives haven’t yet been completely defined.