The most commonly used AAS in medicine are testosterone and its various esters (but most commonly testosterone undecanoate , testosterone enanthate , testosterone cypionate , and testosterone propionate ),  nandrolone esters (most commonly nandrolone decanoate and nandrolone phenylpropionate ), stanozolol , and metandienone (methandrostenolone).  Others also available and used commonly but to a lesser extent include methyltestosterone , oxandrolone , mesterolone , and oxymetholone , as well as drostanolone propionate , metenolone (methylandrostenolone), and fluoxymesterone .  Dihydrotestosterone (DHT; androstanolone, stanolone) and its esters are also notable, although they are not widely used in medicine.  Boldenone undecylenate and trenbolone acetate are used in veterinary medicine . 
As for testosterone; you understand how important testosterone is to the human body; in-fact, it is one of the most important hormones we produce. Through the use of anabolic steroids our natural testosterone production is suppressed and while total suppression varies with each and every steroid, Nandrolone steroids have a 100% suppression rating. Without testosterone supplementation your body will not have the testosterone it needs to function properly; no matter how special you believe you may be or what your friend may have told you, if you supplement with a Nandrolone hormone you wont be making any testosterone. It is important to note when Deca 300 use comes to and end and once it and any other anabolic steroids have cleared your system your natural testosterone production will begin again; however, during the actual cycle it will not exist. By supplementing with exogenous testosterone we eliminate this problem, we provide our body with the testosterone it needs to function and as a bonus only enhance our Deca 300 use as the testosterone hormone in of itself is highly anabolic.