Ok, I have to ask...why did Amtrak do the smartphone angle on this? I definitely get the revamp of the voucher policy, and I also get the shift to scannable tickets (which if nothing else could allow, in the event of a lost ticket, the lost one to be "voided out" and replaced with minimal hassle). I just don't get why "print out a piece of paper at a machine" is so difficult (or why the present system for dealing with un-staffed/un-machined stops was insufficient).
Basically, because it's the 21st century.
As others noted, it allows people to bypass the line needed to pick up a ticket. If someone is running around and making travel arrangements on the fly, visiting a computer terminal and printer isn't necessarily the easiest thing to do (and, many people don't like logging into e-mail accounts using a computer that isn't their own, so unless they're at home or in their own office, printing out an e-mail may not be an easy option, even if they did have access to a printer).
AU probably doesn't have many (if any) people in this demographic, but there are lots of travelers these days who do make last-minute travel plans, running from the office to the airport/train station, booking tickets on the cab ride over. If you get to the station with seven minutes left to departure, are you going to want to wait in line to pick up your ticket, or fumble around with a quiktrak trying to get your reservation?
Whenever I fly United Airlines, I use the electronic boarding pass if available (not yet available on interlined itineraries or international itineraries) and never touch a piece of paper. Never had a problem.
Granted, there are some hiccups at Amtrak, but system-wide eticketing is barely a week old, so there will be some teething problems. But those will be resolved in due time, as everyone gets used to the system. By this time next year, people will be wondering how Amtrak managed to survive so long without eticketing.