That way you can have two sets of network settings ready to go. For example you can set line one to be office A and line two for office B. If you don't need this sort of portability just set both lines to use the default settings by ticking the 'Use Default settings for this line' box.
If you are in doubt about whether Express Talk will do what you want we recommend you try the free download. Seeing the program in front of you often answers most questions