If you look at most the Terran Buildings you will notice that many have the Lift-Off command available. These buildings are capable of limited flight, and while slow and vulnerable in the air their mobility can be used both for rapid expansion and to flee from enemy ground troops. The Command Center, Barracks, Engineering Bay, Factory, Starport, and Science Facility are all capable of flight.

            Buildings may only land on open, buildable land. You cannot land buildings just anywhere. You cannot land it on top of units either, so you cannot crush enemy units with buildings. Buildings have no attack of their own when flying so they are vulnerable to attack, and against most air units they will take even more damage from              air-to-air attacks.

             Some interesting uses for Lift-Off:

                  When a resource spot is mined out, rather than just letting your Command Center sit around, you can fly it to another resource spot.

                  When your buildings are being attacked by melee/ground units only, such as Zealots, Zergling, Ultralisks, Reavers, Tanks, etc. lift-off your buildings. The melee units will not be able to attack them.

                  This can save the buildings until reinforcements arrive, or force the enemy to engage other buildings like Bunkers. This is especially useful with Command Centers as they are very expensive.

                  In island maps, you can build a Command Center and fly it over to an island without a transport. You can also fly Barracks over early in the game and build a few Marines on different islands to secure them before the enemy is able to transport troops to them.

                  You can construct a building on lower ground, then fly it up to the safety of high ground to either build units or to keep the buildings safe. Some maps have secondary resource nodes on high ground that aren't accessible from the ground.

                  If you have a building capable of two add-ons such as the Science Facility and Command Center, and decide you want the other add-on, instead of destroying the old add-on and building the new one, you can lift off and build the other add-on and switch between the two as necessary.

                  If you find that a Terran enemy has abandoned some valuable add-ons, you can fly your buildings to that location, land by them, and use them. This is of course a rare application.

                 If you are being nuked, you can lift-off buildings in the targeted area and run before the nuke hits if you are fast enough. You probably won't escape the blast, but you can get away from ground zero and reduce the damage that you take.

                  You can run away and set up at another location if your town is about to fall. You can return when it's safe, or run away to some other location to rebuild your town. Flee toward allies in allied games.

                  You can lift-off a building and fly it a bit out of your town to spot for your Siege Tanks, increasing their effective range. If your building comes under attack you can always fly it back into town, repair it, and send it back again.