Square Enix is a well known developer and publisher of games for the PC and console.
Today they have grown to become a larger scale publisher so they have launched a new store to offer games.
Steam is expensive for publishers to use so may publishers have established their own stores. Steam may be used for content delivery but some publishers have even gone even further.
The site still has some problems but they are working on it actively so it should be ready for mainstream use in a few months as they fix problems. Web development is complex and it can take a while to get the problems all fixed.