Wondering why Amazon is going to announce a smartphone tomorrow? This chart from Statista provides a little clue.
People who subscribe to Amazon Prime spend a lot more than non-Prime members. Prime is a $99/year service that includes two-day shipping, a video streaming service, and a music streaming service.
Presumably, Amazon's phone is going to have hooks into Prime. Perhaps Prime members get the phone for free or at a discount. Or, maybe just logging into the smartphone starts a trial membership to Prime.
Amazon wants to cut down on the friction between wanting to buy something, and owning something. An Amazon phone connected to its Prime service in your pocket, should make a difference.