There are three reasons that the insurance companies hold some money back.
(1) To make sure that you get the work done.  Insurance companies have learned if they give the customer all the money upfront, some people end up spending it elsewhere and not on the work that needs to be done.
(2) They want you to pay your full deductible and repair your property.
(3) And finally, if the roofing work costs you less than what the adjuster has estimated, the insurance company will pay you based on that lower sum. That saves the insurance company money.  But to get a roof replaced for less than the adjuster estimated means that the contractor is probably going to cut corners somewhere on your roof installation. You should choose to buy as good of a roof as the insurance has agreed to allow for with their estimate.  Legally, you have to pay the deductible either way.  Go ahead and get the best roof you can.