This is obviously part of Amazon's effort to build out their own delivery network in order to muscle out fedex, ups, usps, etc. If their drivers aren't delivering packages they can be delivering food. That makes their whole delivery network stronger because it allows them to have more drivers employed driving around town all the time. It's the same reason Uber got into food delivery.
The problem at the moment is Amazon logistics (at least here in the UK) is terrible. Prime went from a next day, 1-hour delivery window, fully tracked with DPD generally delivered by 1 pm to next day sometime between 8 and 8, no tracking and sometimes not delivered at all for no reason. If a parcel arrives at 8 pm the day after it's ordered it's not a next day service as far as I'm concerned, it might as well arrive…
They also always find a neighbor to accept the package. Royal mail always leaves a card which means I have to pick it up from a post office (which is only open when I work). The main reason I order nearly everything from Amazon is that most other sellers here use Royal Mail 2nd class, which for my address takes at least 4 days to arrive, before christmas often 2 weeks or more.