Speed rating are relatively new in British horse racing compared to form ratings and basically came from American racing. America is very different because all the circuits are almost exactly the same in terms of size and surface. Americans have used speed ratings for many years. This changed when American Andy Beyer wrote his book Picking Winners in 1979 and revolutionised the way people look at horse races.
America is very different in that they have sectional timing, that is splitting a race into four sections and give an exact time for each section. This allows people to tell exactly how a race was run and whether or not it was run at a true pace as well as similar courses. In Britain there is no sectional timing and each racecourse has it's own characteristics which makes direct comparison of times over different courses impossible. For example, you cannot compare the downhill 5f sprint at Epsom with the mostly uphill 5f sprint at Sandown. In addition, you need to take into account the different ground conditions of each race. Speed ratings attempt to factor in all the different variables to produce a set of form ratings which allow comparison of different horses under different conditions.
It comes down to your own personal preference whether or not you use speed ratings and there are a couple of professional speed rating services available to you, such as Topspeed in the Racing Post.