I think that the main purpose is to stop people from canceling Prime so frequently, and the secondarily, it's a deeper level of data for Amazon to tie to your profile. Aint no money in cell phones like that anymore. Just read an article that also hinted at that being an improbable reason for Amazon to get involved. Cell phone plans have been a race to the bottom ever since T-Mobile started switching things up 5-10 years ago.
Amazon are going to rent out spectrum from the big phone companies, and sell it for the low just like MNVO's (Mint Mobile/Boost Mobile/etc) already do. If I was AT&T or Verizon, I would turn them down. The more customers that realize that 99% of the time, there's no difference between post-paid (plans thru Verizon/AT&T) and prepaid MNVO plans (plans thru companies leasing bandwidth from Verizon/AT&T), the more it's gonna eat into the big companies' pockets. People won't take a chance on Boost Mobile to save $$$, but they'll take a chance on Amazon
EDIT: Did some more research, and Amazon might be partnering up w/ Dish, who will be the new kid on the block when it comes to mobile spectrum. They are playing disruptor. It's gonna shake up the mobile industry a lot potentially. No wonder all the other providers were down a lot today.