Your dog could have sores for various reasons. He has a wound that is healing causing a small sore on his body or he's created a sore through hotspots, itching and scratching. Maybe he's gotten into something you're not aware of that caused a sore on his skin. Either way, you'll need to keep it clean to ensure it does not become worse.
If your dog has been injured by another animal resulting in a bite or puncture wounds or even a scratch, it's important to get the sore cleaned up so you can monitor it and ensure it is healing quickly and correctly.
If your pup has sores on his skin, you can bet that area of his skin is probably pretty tender. He may flinch when you get near his skin so just be cautious and aware of the potential pain he might be feeling. Be gentle and consider how to make him more comfortable.