It's a gray market site. That means they get their keys from sources other than the publishers themselves (as sites like Steam, Greenman Gaming, Amazon, Gamefly, and all of those do).
These "sources" are most often Russian / Eastern European stores, where games are sold more cheaply due to the lower income of those regions. These sites will buy keys for cheap there, and then sell them to Western consumers and pocket the difference. This is one reason why many times your "key" is the picture of the back of the box or CD case. Other sources include: Stolen keys, keys purchased from bundles, and keys purchased via stolen credit cards.
Lots of people buy from them and have no problems. It's extremely caveat emptor though -- if you do get ripped off, or if Steam/Origin/Uplay ever decide to crack down on this (like allowing keys from Russian regions to only play in the Russian language), don't expect any sympathy from anyone.
Additionally, if you're the type who actually cares about where your money goes / who you support, you're not doing the developers/publishers any favors from purchasing from those sites. Again, keys are purchased/acquired for dirt cheap and re-sold to the West, so most of the money goes to the site itself.
The sole purpose of this space is to make my post look longer.