Proper record keeping is important because it can be used as proof of the conception date - a.k.a. The conception date is also important when an interference is initiated to prove which inventor was the first to create the invention.
Detailed records are also evidence that the inventor was diligent in developing the invention.
It should be noted that a prototype is not needed for proof of the conception date. The conception date may be relied upon as the date of invention for determining patentability and priority only if the inventor exercised due diligence in reducing the invention to practice. This is accomplished by the inventor exercising continuous activity from before the reduction to practice date of another invention to the date of reduction to practice. Continuous activity does not mean the inventor has to work on the invention every waking minute. To establish diligence, the person must account for the entire critical period from the time just prior to another's conception.