Your dog may have a healing scab or a scab as a result of a rash or sore. Some dogs tend to scratch until their itchy spots scab over. There are many reasons why your dog may have scabs such as food allergies, cuts, and injuries or even just a bad habit of scratching incessantly. If you notice your dog's skin is starting to scab over, you will need to keep an eye on it to ensure your dog does not continue to break it open and that the wound beneath the scab is clean. It may also be a good idea to try to understand why your dog's skin is scabbing and address that issue as well.
If your dog is constantly scratching his skin causing scabs, then he's probably pretty uncomfortable with dry, itchy skin. If he has an injury that has scabbed over as part of the healing process, it's probably good to know that he may be feeling a bit better. Either way, keep a close eye on him as this scab heals to ensure it does not create a larger problem.