As a user of Greggs and McD (UK) I can maybe offer some insight.

Fairly obviously the discounts are to encourage customer loyalty so you keep going to McD rather than somewhere healthier. Also to get you to come back - if you haven't been for ages they may offer a 99p big mac to get you back in.

As to why apps rather than paper coupons, my closest McD has a typically had group of about 20 people waiting for the 50 items they ordered with new stuff ordered every 30 seconds or so. The last thing the low wage rushed staff need are customers going can you explain this coupon to me an is it valid for extra fries on Friday etc.