There are several services and online websites that require users to have two-factor authentication. A good example would be banks where you might be required to key in a one-time code to verify you are who you say who are when making purchases online, or when trying to transfer funds to someone else.
That being said it looks like WhatsApp could be gaining that functionality soon. According to the latest beta of WhatsApp, a series of files has been discovered inside the APK that make reference to two-factor authentication, although exactly what this will be used for remains to be seen as the folks at The Android Soul have not found anything in the beta app itself that would need the feature.
Now since WhatsApp requires a code when being installed for the first time, or when you’re installing the app on a new phone, it has been speculated that maybe the two-factor authentication could be used for this. However the current system already works so we’re not sure why WhatsApp would change that.
It is possible that maybe this is for when trying to retrieve backup chats, and WhatsApp wants to make sure that you are the owner of the phone. In any this is just speculation but there are some references, so maybe we will find out more about the feature in future builds.