Max Area of Island shows that DFS can do more than mark cells. By returning a count from each recursive call, you turn DFS into a measurement tool. This "DFS that returns a value" pattern appears in tree heights, subtree sums, and longest paths.
Look back at the grid problems you've solved. Flood Fill changes connected cells. Number of Islands counts components by erasing them. Island Perimeter skips DFS and uses a local scan. Max Area of Island adds computation to traversal.
Each problem added a new twist to the same DFS-on-grid template.