The work of Roy Kerr is today fundamental for approaching issues of relativistic astrophysics as diverse as the Gravity Probe B mission of NASA arround our planet earth, the Binary X-Ray sources in our galaxy, the active galactic nuclei in the cosmos. No astrophysicist can work on these issues without recalling daily Kerr work. Essential to the utilization of the Kerr solution has been the separability of the Hamilton Jacobi equation by Brandon Carter. I recall how starting from the work of Carter, by introducing with Wheeler an effective potential technique and the concept of "ergosphere", we obtained the Christodoulou-Ruffini energy mass formula of the black hole. The concept of extractable energy of a Black Hole, implied by this formula, plays today the fundamental role in understanding the energy source of gamma ray bursts (GRBs). The basic energetics of GRBs needs the introduction of the novel concept of a "dyadosphere" of a black hole. Reasons are given why the occurence of a dyadosphere within the ergosphere of a Kerr black hole can be the key factor in understanding the sstrophysics of jets in microquasars and active galactic nuclei.