Fork seals should be replaced if you notice that they are cracked, or if you notice that they are leaking. When fork seals are leaking it could be some fluid on the fork tubes, or if they are really bad it could be actively running down the lower legs. You need fluid in your front forks for them to operate properly. The fluid is there to allow some control of the compression and rebound of the forks. It is also there to keep everything cool while the forks are in motion. If you loose too much fluid, you can seriously damage your forks, or cause handling and stability issues.