Square Enix is introducing Steam account linking to Final Fantasy XIV to "further enhance account security and bolster fraud prevention measures". The change will come into play early next month, at which point you'll be prompted to link your FFXIV account to your Steam one if that's where you hang your hat.
To link your account, you'll need to possess an FFXIV licence that matches the one found on your Square Enix account. You can verify the licences by clicking on your Steam display name, selecting 'account details', and then clicking on 'view licences and product key activations' under the store and purchase history. If you're looking to find out what type of account you have, you can head over to the Mog Station and select the 'service account status' menu, where you should see a Steam notation if it applies to you.
Currently, you can use your FFXIV log-in to play on anyone's Steam account regardless of whether it's yours or not. However, once the change comes into play, your FFXIV log-in will be tied to one Steam account.
RELATED LINKS:
FFXIV is getting another Deep Dungeon
FFXIV's Valentine's Day event returns soon - here's when and where
FFXIV housing - prices, locations, and patch 6.1 changes