They are just there to aid perspective and a sence of movement. Some people also consider them a sign of a gravity well, which is sort of true, but not a very good indication.
Yeah sometimes you cant see the actual planet if you're facing its dark side so they are helpful in that way plus they can help to indicate its size. I know many people say it indicates the gravity well but I'm unconvinced of that.
The blue circles are scaled up versions of the planet radius and are there to give you an indication of how quickly you are approaching. Many say they are gravity wells but a planet doesn't have multiple gravity wells that increase in size