Likely the players do not know the song by heart, and are reading the music instead. You still have a conductor if everyone has the whole thing memorized, though, to keep everyone together. By the time you are performing, the conductor is mostly ornamental – they do keep the beat, but if you’re a professional musician and you don’t already know where the beat is, you’re hosed. Most of what the conductor does is rehearse the orchestra. The musicians will read and practice the music themselves, and then the conductor brings them all together and had them practice important or difficult passages, and explain how they want the different parts to fit together, or stylistic changes they want. For younger musicians, they may also rehearse difficult passages to help the musicians practice, but as you get older and more skilled, you’re expected to do that on your own. When you go to watch your local symphony, the conductor is basically dancing and it’s more about conveying emotion and pulling the tempo faster or slower.