The main point is, you can see planet systems at the beginning of the game (with stellar cartography) but not the quality of the planets. So you set out to explore a sector stuffed with planets, hoping there is a bunch of high quality planets, but you just might find a bunch of worthless ones. Meanwhile, a system with only two planets might have two great planets. So you set your exploration priorities with the number of planets in a sector in mind, but the game might throw you a curve ball by making what appears to be a rich sector near worthless, or a poor one valuable.
Just makes the initial exploration period more exciting and fun, seeing how lucky or unlucky you get. Once the map is cleared, yeah, they are just eye candy. But it still makes the game more realistic, as a lot of planets in the universe will be worthless to any space faring civilization.