For me, it's also the ease and simplicity of being able to use a single payment system.
Ditto. Amazon makes it so easy to buy.
Amazon very rarely has the lowest prices on anything if you shop around. Books, for instance, are usually cheaper on Overstock. I have a Prime membership nonetheless, due to the convenience and incredible customer service.
But without having to shop around I know they are near the lowest. Within a few clicks I can have it on my door in a couple of days--no time wasted checking around. I buy tons of stuff on Amazon (literally the jeans I'm wearing now to the pans I cooked dinner in).