Wet and fresh food preparations have several benefits for a dog that is having trouble gaining weight.
First off, they are often very dense in nutrients and calories especially with the fresh food options. This is because there is very little filler, if any, and so all that your dog is consuming is good for them.
There is also the fact that this food is easier to chew and therefore ideal for dogs with dental issues. Finally, wet and fresh foods simply tend to taste a lot better and you are more likely to get your dog to consume them if they are being picky.