As Scuzi has said - Avoids you levelling off at 80 when you could get further descent shortly. Usually done if there is an outbound under you at 6A and we need to get you going down for a CDA. Sometimes also happens when you're on the trombone from LAM - If we turn you round the corner downwind and we're turning you within spitting distance of another inbound who is just out of 6A, dropping you to 7A gets you going down. (You should then be cleared further before levelling off). It isn't seperated from 80, so obviously isn't used if you're near anything at 80.
It can be used in lots of different situations, but those are the main 2 I have used it for (the few times I have).
Hope this helps.