A trombone's slide is a precision-fitted pair of tubes that can bend or lose its smooth action if the case gets jostled with the slide unlocked. Locking it and packing slide cream separately are the two things that keep it playing the same after the move.
Pro tip: A very light, even coat of slide cream applied right before your first play at the new home restores smooth action faster than assuming it will feel the same after sitting in a case.
Watch out: Never move a trombone with the slide unlocked and loose in the case; repeated jostling against the case walls can put a dent in the inner slide tubes that ruins smooth action.