A scanner is sent to a planet to take a picture of the surface of a planet and email it to you. This picture will not tell you the levels of the buildings however. For that you need the Surveyor which also provides a picture of the planet.
Scanners are also useful as the first wave of any attack. Since they are cheap, you can flood a planet with scanners before the main attack and cause the defender to waste drones shooting them down instead of your more expensive ships. Scanners are also frequently employed to get past SAWs in a planetary system. If timed right a wave of Scanners go arrive just before a Stake or a Colony Ship causing the SAWs to go into their 5 minute cool down period allowing the Stake or Colony Ship to arrive unharmed.