First of all, barbell exercises require few and relatively inexpensive equipments. Unlike gym exercise for which you need special equipments for each exercise you can perform most barbell exercises using a simple barbell, a set of weights and a workout bench. This also means that since barbell exercises take little room to perform, they can be performed anywhere; be it at home, at the gym, wherever.
Additionally, barbell exercises are very good at exercising stabilizing muscles on top of the main muscle groups. Since you are not stabilized in the same way you would be if using special gym exercises, stabilizing muscles are required to perform the exercises. This feature is helpful in making sure little muscles are not left behind are allowed to keep up with bigger ones.