For example, middle (but not anterior) deltoid activation tends to be greater during free weight bench presses compared to Smith machine bench presses (Schick et al. 2010). (strengthandconditioningresearch.com)
Both groups performed free weight bench presses three days per week, but the first group trained continuously throughout a 24 - week period and the second group performed two cycles of a 3 - week detraining period followed by a 6 - week retraining period. (fitnessandpower.com)
Take the bench press for example, there was a study comparing smith machine bench press vs free weight bench press: (getthisstrength.com)