Say I have an idea for a robot, and I need a few dozen battery clips, some NiMH batteries, a dozen servo motors, a bunch of disc magnets, a box of M3 screws/nuts, some PLA filament, a microcontroller board, and a small display panel to show status/errors/etc.
I can buy all of that and have it delivered in a day or two, for less than $5 more per item than I would pay if I ordered it from a Taobao listing and waited on international shipping. And I tend to have less problems with the Amazon listings, presumably because the sellers know I can return them quickly and painlessly if there are quality issues.
There is a huge benefit to being "the everything store".