Meno, consider what you're doing when you're doing pushups. You're stressing your body with weightloads foreign to the body. If you do biceps exercises but no forearm or compound arm exercises, you'll have a severe imbalance.
I don't know who told you that bodyweight exercises means zero weight. If that were the case, have people just been chasing their tails wasting time with calisthenics? Some people do nothing but calisthenics, but their tissue becomes denser and thus gain muscle. You need to strengthen your whole body and compound exercises are just the way to do that if your body isn't prepared for the maximal stress that actual lifting puts on it.
