The reasoning behind tapering is that by lowering the dosages slowly, your HPTA would be slowly normalizing so that when you were completely off you would be producing natural test. At the time, they did not have the post cycle drugs we use today to re-establish testosterone production.
This theory is flawed because nearly any level of supraphysiological testosterone or anabolic steroids will supress natural testosterone production. With the use of drugs like nolva, clomid, and HCG, there is no need to do this.