This is so important many people don't realize. I will never buy another new vehicle without it, it will be a condition of the sale. Aftermarket repair data when you can find it is never as complete.
Since 2002 I have been going round and round with Mastercraft for a diagnostic manual on our boat and even though we are out of warranty they are not budging.
Even if you never do your own maintenance its important, in CD form you can toss it in the glove box on trips so someone with basic knowledge and that CD can get you going. For non warranty stuff your local shop might like to burn a copy as an OEM backup to the aftermarket stuff they buy that covers many makes & models.