I find Primer interesting for the relatable human story. What are they doing with this knowledge and why?

Even Primer is fairy dust. The plot makes enough up to suspend your disbelief. For instance, I just made a rule up for my hypothetical new movie: travelling through time drops the traveler in the same location of the frame of reference they inhabit. This is always a fixed location relative to the Earth if the traveller is static relative to the Earth, and hence in the same frame of reference.