Studies show that compound movements often result in similar levels of muscle activation when compared to isolation movements – this specific study found that the bench press had similar levels of chest activation when compared to dumbbell chest flyes. The bench press also caused this activation over a longer period of time than flyes. This means that, especially for beginners, isolation exercises are not necessary in a training program.
Reference: Welsch, E. A., Bird, M., & Mayhew, J. L. (2005). Electromyographic activity of the pectoralis major and anterior deltoid muscles during three upper-body lifts. Journal of Strength and Conditioning Research, 19(2), 449.