Because it weakened the Japanese morale. The IJN would no longer be capable of operations in the Pacific Southwest because their resources were low. They were also still reeling from the losses at Midway.
The Americans secured Henderson field, a major airfield that prevented the Japanese from spreading south into Australia. The Aussies would be helpful in Operation D-Day.
US-Australian supply lines were no longer under threat. It's why the IJN tried to take the island in the first place.
