In a nutshell
Merge Intervals fuses overlapping ranges — like combining busy slots on a calendar — into the fewest blocks. Sorting them by start time means any overlap can only be with the block you're currently holding, so a single left-to-right sweep is enough: if the next range overlaps, stretch the current block's end; if it doesn't, the current block is final and a new one begins. The cost is dominated by the sort.