Now that everyone has email it isn't as practical anymore though. People forget or lose receipts, while it is convenient to print at home.
Not everyone wants to give out their email, though.
There are small businesses that use that Square thing on their cell phone to get paid, and I could imagine printing a receipt off your phone (especially if they're a plumber or cable repair guy and are at the customer's house), it would make sense.
Just got a speeding ticket...something like this already exist, where the trooper type on the smartphone, while the printer extension printing your ticket.
I don't find any practical use to this prototype, to be honest: For a business? A receipt printer is cheaper, both the machine itself and its accessories. A domestic use? Unless you need to print something quickly or in the act, a normal printer would be cheaper. The only plus would be the portability, but we have the Polaroid Zip already and it really haven't take off as a popular product.