The purpose of this field is to allow the app to discover whether the user purchased an early version or a late version. Specifically, to convert an app that is originally launched as a paid app to a free app with IAPs - here's how you do it. You check this field and if it indicates the early, paid version you credit the IAPs. What I do is simply use a build numbering system with a dot. My early paid apps had all kinds of build numbers - frankly I don't recall them - but they did not start with a "12.". My newer versions all have build numbers that start with "12.". So all I need to do is check for a "12." - if it's not there the original app was a paid version.
Yes, it would have been more direct if this field contained the app version not the build version. There must be some reason why Apple was unable or unwilling to do that. But whatever the reason, this works.