Overview example: imagine that you have many segments of dialog on tracks A1 and A2. You edited them all in before adding other audio elements (for example, music). You have meticulously adjusted each segment of dialog to what you felt at the time was just the perfect level. Later, you added some music, ambience, and other sound effects and now realize the following:
The various levels you set for the dialog segments are still perfect in relation to each other (meaning that all the actors are at an even level, one actor is not disproportionately louder or quieter than another)
Now that you've added the other audio elements (music, and so on.), you need to adjust the dialog segments a bit louder without losing the relationship between them which you've already established