I was first introduced to foley sound by dexter fletcher at a film seminar held to help us students that are looking to create a good opening sequence, an edge and skill over others. He explained that it is added in sound to a sequence that microphones do not always pick up on when filming. This can mean the difference between a clear sound affect of a pin dropping to make the video more realistic.
Here is a foley artist called Caoimhe Doyle that was nominated for an Emmy award for her work, demonstrating how she makes the sound affects and how important they actually are to a film ;
I find this video so helpful explaining what foley is, and actually understand how much it is used in films…from lips kissings, to the sound of a bone breaking.