Thread Cutting

In this video a line in cutting profile and a line in part is selected to make both collinear then Helix/Spiral and cutting profile is selected to cut a thread.  

My question is, what is reason for making collinear?

SolidWorks tutorial | Design And Assembly of Pipe vise in SolidWorks - YouTube

Time 55.03-55.12

SolidworksSketching