Flat-ironing helps ruffled hair cuticles lie flat, which creates more surface area to reflect light

• Healthy cuticles will lie flat on top of each other.  In so doing, they increase the shine of the hair. Note however, that if hair is not shiny, it doesn’t mean that it’s not healthy. For instance, healthy straight hair has greater shine than its curly counterpart because the hair fibers are better aligned against each other. Without cuticles, hair coloring would not even be possible.  They behave as ‘gates’ which open at high pH and close at low pH.   High pH causes swelling of the fiber causes the cuticles to ‘flare’.  The hair dye precursors can thus penetrate into the cortex.  As the pH is lowered, the cuticles get more aligned flat and the big dye molecules get trapped inside the hair

http://hairmomentum.com/importance-healthy-cuticles/