Its main purpose is safety. As a visual marker to show the steps are. I'm finding this especially useful for my toddler. The lines are a very useful guide for him. He can see them underwater and knows that is the way to safety.
They also tend to be a grippier tile, same as anti slip strips on a normal staircase.
Edge markers are requried for safety on commercial pools and recommended on residential pools. People may not be wearing their glasses or have water in their eyes while going in and out and it helps visibility.