Although I agree with most of the above I would like to add the following. A full alignment will make the IR's seek True north to provide heading information (as well as map information after full alignment) and a Level platform to provide attitude information. If at the gate after landing a drift is noticed (ie sustained groundspeed) a quick re-alignment is recommended which gives the ability of 1) Zeroing all groundspeed and 2) Entering a new present position.