Generally, when you need to re-sheet, re-deck, overlay, or replace some of the wood under your shingles it has to do with proper installation and building codes. Older homes sometimes have gapped boards that were a common practice when it was built, but building codes have been updated since and require that you cover that gap to allow for proper installation. Installing shingles on gapped boards can lead to them not being nailed down well enough. This may cause your shingles to blow off in a high wind or even leak easier. Also, if your roof has been the victim of dry rot causing the deterioration of the wood, it may be required to do a re-sheet or an overlay of plywood. This can ensure that your shingles will be securely nailed to you roof and prevent any unnecessary damage to your roof. It may be found that only some of your decking needs to be replaced. In this case, we would typically recommend that only those damaged boards be replaced.