Without it, you will have difficulty establishing the date on which the invention was made, and then you may lose your patent priority even if you truly were first. Pre-date any prior art . Your invention notebooks can be your defense if accused of taking a trade secret from a competitor, as your invention notebook can show that you developed the information, or already had it before you ever had access to the adversary's information. Defence if another company receives a patent first. Just because you're not going after a patent doesn't necessarily mean you don't need this evidence.
Wilful infringement carries severe penalties.
Or, you may be able to prove prior published invention, thus completely negating their claim against you. Either way, the better your invention documentation, the better your defence. The hardest part of the whole patenting process is pulling together the documentation. Usually by the time your company is actually in filing mode, the invention process is a distant memory. Having kept diligent notes all along can turn this gruelling exercise of assembling your evidence into a no-brainer.