Hi,
HOW : In frequency response analysis the exitacion is explicitly defined in the frequency domain, as opposed to trancient analysis - time domain. Thus, all of the applied forces are known at each forcing frequency. Forces can be in form of applied forces and/or enforced motions (displacements, etc).
WHY : The engineers want to know the structural response to various steady state oscillatory exitation, e.g. from the crank shaft. This exitation should be of course with long duration, this is normally the dynamic cases in Automotive or Aeronautic. Its result is a curve over the frequency. From this curve the peak response can be obtained and the stress on this frequency can be analyzed. Unfortunately i don't have any idea for utilizing the results in automotive discipline, may be our friends from automotive could help us futher.

regards

Thanks for the information
Sasank