Walking a dog with a harness is safer and more comfortable than walking a dog with a traditional collar. However, many dogs are afraid of having a harness put over their heads. Shelter dogs are often already stressed, so wearing a harness can easily induce more fear. By creating a positive association with the harness (or any new equipment), you can help a dog overcome his/her fear.
Targeting, when a dog to touches a body part to a specific object or person, is a very valuable and versatile skill to teach a shelter dog, as it serves as a base for so many useful behaviors (come, heel, go-to-crate, go-to-place, say hello, and many more). Targeting can also help many shy dogs develop confidence with people fairly quickly.
This video is an introduction to targeting, specifically nose targeting!