They are often used on matted dogs to leave the hair a bit longer than a regular (finish) blade. They are extremely dangerous to use if you are not crazy careful due to the large spaces between the teeth. I am a professional groomer and I do not own a skip tooth. If a dog is that matted I would rather just shave it off short.
It can be used for blending between shorter and longer haired areas, but I just use scissors and thinning shears to do that.