Why do coffee shops ask for phone numbers?
Discussion
npubs are too long
Rewards programs or if you want a mobile receipt. Always a good idea to avoid. 
To let you know which businesses don't deserve your money.
The worst is when the point-of-sale system starts to ID you by your credit card number and then, if you’ve ever given a phone or email to any merchant using that POS system, you get pinged after shopping SOMEWHERE ELSE to set up a rewards account….