Targets can be a versatile training aid. Dogs, cats, and other animals easily learn to touch a target for a click and a treat. Touching a target is also an easy behavior for new clicker trainers to teach.
Shaping and Targeting
Random or variable reinforcement is a useful procedure in making a given behavior resistant to extinction, for example in the shaping process, when one wants to raise criteria. To go from reinforcing every response to selectively reinforcing stronger responses you need to develop enough resistance to extinction so that the animal neither changes the behavior instantly upon going unreinforced once or twice, nor quits altogether. Resistance to extinction is also important in maintaining long duration behaviors, as in searches, field trialing, and so on; and can be developed gradually. Bob and Marian Bailey might consider this simply another example of a shaping schedule.
"Come" is no harder to shape than any other behavior BUT in real life it has a huge component of criteria to raise. Start indoors. Use a clicker and desired treat, not kibble, for several one-or-two minute training sessions daily. Call the dog, and click if he comes toward you. Do this in your living room. Call him from a few feet, and click, when he takes one step, then more steps, of if he comes right to you. Then call him back and forth between two people. Click and treat good responses. Ignore poor responses. If you get more than one or two poor responses, retreat to an earlier shaping step and reshape upwards; this just means you don't have the behavior at that criterion level yet.
From Chapter 2 of Don't Shoot the Dog by Karen Pryor.