It was important because the Iron Bank was about to shut Cersei down, and now she should be able to pay them off.
But strategically,
look at the map. Highgarden is pretty useless down in the south-west. It was never fortified, because nobody wanted it. Danny is perched just off-shore of King's Landing. Dorne is to its south. Littlefinger (for now) is on board and holds The Vale. RiverRun should revert back to the Tullys. The Twins are up for grab. Winterfell is back with the Starks.
The Lannisters just retreated entirely out of any position north of Kings Landing. They've got the Iron Islands with Euron I suppose, but it's useless. They are bottled in.