I have it and love it. There's definitely a risk of unnecessary purchases, but I find that to be more true with impulse buys in a store than online. I'll often keep a running shopping list in my cart and then wait to check out to make sure I really need something.
Here's why I'll renew again:
-- Better selection. I can find ANYTHING. I am doing a lot of home remodel projects and can't tell you how many times I've been to Lowe's looking for a specific thing and the clerk has told me they don't carry it, "check Amazon." ARGH! But yeah. I try to shop local brick and mortars when I can, but sometimes it just doesn't work.
-- Free shipping. As someone above pointed out, it's not exactly free since they make up the price differential, but having items delivered to my house, particularly when there's a very different set of things I need, makes my life so much easier than running around to 10 different stores on the weekend. And when I need something fast, Amazon's two day shipping option is usually faster than I can get to a store anyway since I work more than full time.
-- Prime Videos -- I love the streaming options. The selection isn't always the best, but I can always find something I want to watch for free OR ...
-- Digital Credit for regular shipping. It's rare that I use Prime's 2-day shipping option. Sometimes I need things fast, but almost always I can wait a couple of extra days and instead get a $1 digital credit for movies, music, and Kindle. This alone makes up for the subscription fee.