A couple of years ago while flying a 757 to Osaka, Japan, we discovered a leading edge slats problem as we prepared to land. We told the controller that we may have a problem and needed time to sort it out. It wasn’t a big deal, we were able to get the aircraft in a normal landing configuration.
The next thing we did was tell the controller we were ready for the approach and… we declared an emergency. Our aircraft had lost important functionality and we were using a back-up system. Control of the flaps and slats was limited and we didn’t know exactly why the slats didn’t originally extend or if the problem may have damaged something else. We fully expected the landing to be uneventful, but just in case, we wanted the runway all to ourselves. We wanted to make sure the controller understood that we might need extra time on the runway and that a missed approach could complicate matters. Declaring an emergency makes all this very clear to the controllers. With fire trucks watching from the taxiway, we made a perfect landing and taxied to parking.