Device Line vs additional lines


    Maybe I am not understanding this feature.  On the DIGITS app (iPhone 6, iOS 10.1.1) , I have a 'Device Line' that's Always On ... GOOD and it's also asking me to swipe to activate other lines in order to continue.   Why?  If I just want my device line to work on my Work phone without an additional line, how do I accomplish that? 


    What's the purpose of the additional line?

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