An iPhone screen also known as an iPhone Display Assembly is comprised of multiple layers.
An iPhone screen needs to be replaced when either one of the two components has sustained damage:
The original first 2 iPhones came with two screen components installed separately.
All other iPhones come with the two screen components fused together as one complete assembly making them impossible to repair individually. Apple has decided to do this for several reasons:
With a fused touchscreen and LCD there is never an issue with dust getting in between the layers of the screen
The screen is thinner, as there is no air gap between the digitizer and the LCD. This allowed Apple to make iPhones a little thinner overall
In our experience we have learned that the primary reason for iPhone Screen Repairs is broken glass, followed by LCD only broken, and in the fewest cases we have seen screen failure without any visible damage or breakage - see the breakdown by failure in the following chart: