Generally, the main difference between free weights and machines is the use/non-use of stabilizing muscles. When you do any free weight exercise, you have to work to keep the weights on the correct path, whereas machines have fixed paths.
So in your case, if you did a proper Bench Press, you can imagine having to make sure the barbell travels the correct path from/to your chest, without traveling to far to the front (it lands on your stomach) or back (it lands on your face) or tipping over to one side.
With the machine Chest Press, all you have to do is press. If the machine is not uni-lateral (meaning both handles move together), you can even make up for a weak side with the other, promoting imbalances.
This makes free weight exercises more complicated, more "functional" and recruit more muscles. In turn, machine exercises enable you to focus more on the desired muscles.
A web search along the lines of "free weights vs machines" or maybe this article, which I only skimmed over, could get you a more in-depth answer for sure.