Why do companies like Stewarts have things like milk clubs and other loyalty programs?

Because they work!  Let’s say that you could buy essentially the same milk or coffee or gas or airline seat someplace else for the same cost.  As long as you feel like the place you are getting it is taking care of you right (treat you with respect, don’t leave you on the tarmac for five hours, stuff like that), everything else being equal you will tend to use the same provider because you (and I, and everyone else) are lazy and creatures of habit.  If a company can then only spend a little bit to get you to use their stuff religiously as opposed to someone else’s, it makes good economic sense for them sense even though they are getting a slight decrease in profit margin, they make it up in volume.

My local coffee shop (Coffee Planet) has a loyalty program that is pretty typical: buy 10 cups get one free.  I swear I get a free one almost once a week!  These benefits can really add up if you actually pay attention to them.