The little circular lugs sticking out of that adapter are grounding tabs. You're meant to connect them to an electrical ground (which is sometimes possible in the US by screwing it into the faceplate of the wall socket). That lets you safely use 3-prong devices in a house that only has 2-prong sockets.
Of course, lots of people mis-use them, and when they do they're unsafe. I assume they're still on sale because they have a safe and appropriate use.
EDIT: I, personally, would never have one of the linked adapters in my house, simply because the risk of someone being an idiot with it without realising is too great. (I also come from the UK, where our plug and socket designs are nothing less than straight up paranoid when it comes to safety, and the US electrical setup seems unnecessarily risky to me. At least it's lower voltage...)
By the way this american type of plug looks pretty dangerous as it must be easy to acidentally or intentionally touch the pins while inserting it into a socket.
Of course, lots of people mis-use them, and when they do they're unsafe. I assume they're still on sale because they have a safe and appropriate use.
EDIT: I, personally, would never have one of the linked adapters in my house, simply because the risk of someone being an idiot with it without realising is too great. (I also come from the UK, where our plug and socket designs are nothing less than straight up paranoid when it comes to safety, and the US electrical setup seems unnecessarily risky to me. At least it's lower voltage...)