It is not a legal document but is valuable, if properly organized and maintained, since it can help establish dates of conception and reduction to practice.
The notebook is also a valuable tool for the inventor since it provides a chronological record of an invention and its reduction to practice.
If an inventor ever has to go to court to prove that they were the first to make the invention, then the witness would be called to the stand to testify that the signature is theirs and they signed that page on that date.
The need for an inventor's notebook will diminish in the future as the United States is progressively implementing a first-to-file system pursuant to the Leahy-Smith America Invents Act.
It has been said that first-to-file eliminates a troubling source of litigation, particularly for individual inventors who may lack the processes and legal resources to defend against evidentiary challenges by large corporate research organizations.