This study found that Smith machine bench press offered similar chest activation to free weight bench, but stabilizer muscle activity was much higher in the free weight bench. Free weights should be the primary focus in your training for maximizing overall development, but machines can be used to focus more on individual muscles in a compound movement.
Source: Schick, E. E., Coburn, J. W., Brown, L. E., Judelson, D. A., Khamoui, A. V., Tran, T. T., & Uribe, B. P. (2010). A comparison of muscle activation between a Smith machine and free weight bench press. The Journal of Strength & Conditioning Research, 24(3), 779-784.