I got it as some sort of a promotional thing way back in the day. It was free at the time. I'm not actually certain if that promotion was supposed to continue in perpetuity, but I'm not going to inspect this gift horse's mouth too closely.
I did recently think, as Amazon box after Amazon box arrived around Christmas-birthday season (all three of us have birthdays within about a month), that I'd be willing to go ahead and pay for it if I had to.
Their free shipping (with Prime) plus fast delivery is really useful.