They offer discount cards as an incentive to customers so that they can better track their customers' spending habits. Unless the discount is substantial, I usually decline their little programs... and tend to not shop at stores which charge one price for "members" and another for non-members. It is none of anyone's business how often I purchase Pepto-Bismol or anything else for that matter. It is bad enough that my prescription medications have to be in a database for everyone and their brother to have access to; one has to draw the line somewhere. If you do use their incentive program, I can assure you that once you purchase something for a specific complaint, you will be inundated with offers and coupons for related products until the end of time.
Medication is usually cheaper in Walgreens when possessing an identification card since this gives the privilege of being a member and thus a major beneficiary of the scheme. This is basically a type of medical insurance plan that calls for registration through the acquisition of an identification card. You should consider visiting their official website for more information.