I have wondered this when looking at pictures of the concorde and sometimes it was down and other times it was up and why cant the commercial airplanes have this system.
When the Concorde is in supersonic flight, the shape of the fuselage has to be very aerodynamic. On landing, the attitude of the plane is very nose high, so the nose comes down to allow the pilots to see the runway.
For any aircraft, the slower you fly, the more the nose is up. For a delta-wing aircraft like Concorde it's even more so, and supersonic flight requires a very slender fuselage. This means during landing and takeoff the nose will block the view forward, and lowering it allows the pilots to see in front.