It's a combination of things: marketing (difficult to quantify, but > 0), interest/appreciation on the float (4-10% annually), forecasted overspend (30-40%), and breakage (5-10%).

The GC face value is a liability on the books though. It's treated as debt when doing cash calculations.

They actually do want you to use the cards though. The overspend is more valuable to them than the other disposition possibilities. Recognized revenue is always the best outcome. The interest/appreciation is the same for the merchant, whether on float or on revenue, but revenue is better for reports.

More broadly: All benefits of the cards definitely accrue to the merchant. There's absolutely nothing valuable to consumers about the deal!