BPs are very real-world necessary. Regardless of how closely the actual results end up dovetailing with the initial BP forecasts, it's the preparation of the BP that forces the biz owner--when it's done thoughtfully and diligently--to give a hard look and critical analysis to those questions that must be answered before launching the venture.
Besides that, no banker, lender, or investor will sink a wooden peso into a startup without first seeing in black-and-white that the entrepreneur has already given careful consideration to all the important aspects--e.g., competition, cash flow, growth projections, market and industry outlook, capital needs, etc.--and who has already foreseen and addressed the likely challenges the new biz might face.
The value of a BP doesn't come from its degree of accuracy in reading the tea leaves. The value is in the structured discipline its preparation imposes in making the biz owner aware of all the questions and hurdles that must be considered--and answered--before the launch.
...it was early and I was full of no coffee...