The "unused" lines are your faction info fields. Your faction leader or someone with a high enough privilege level in your faction can set information there to keep track of your position, rank, fleet, or similar things. The second two lines only show up on the private faction memberlist, and the top line will appear beside your name when you make any sort of transactions, if it is in use. So if your top line says Sergeant, you would see "Sergeant Dominic Mort has sent 20 credits to Syn."
The signature is only used in these forums. You can see mine below, assuming you do not have signatures disabled in your account settings.