If all stars rotate, why was there a theory developed that requires non-rotating stars? According to Penrose's research, a non-rotating star would end up, after gravitational collapse, as a perfectly spherical black hole. However, every star in the universe has some kind of angular momentum. 

Why even bother doing that research if that won't ever happen in the universe and does it have any implications for the future of astrophysics?
In a similar way, we could ask...


  No beams can be exactly 1 meter long. No beams can be exactly
  straight. The material making up a beam cannot be truly isotropic.
  So why should we bother calculating the stress in a 1 meter straight
  beam having isotropic material?


Because knowing how to perform this calculation is a building block for doing more complex calculations.

The non-rotating black hole calculation also provides a limiting solution.  The solution for a spinning star's collapse will approach this solution as the spin approaches zero.  

Similarly, Newton told us that as external forces approach zero, the path of a moving object will approach a straight line.  This is useful to know even though there is no place in our universe that doesn't have gravitational influence.