That's their entire point.

Extreme price discrimination against people who don't know better/don't bother/etc.

McD has the most generous coupons in their apps among fast food chain and there is basically no limit.

It depends on the market. I worked at the East Palo Alto McD while I was getting back on my feet a few years back and we had hours where 60-70% of the orders used the app in some way. I’d say probably 35-40% of our customers used the app; of regulars, more than 50%.