For some jobs, the time it takes to copy your configuration and spin up the machine per task takes longer to complete than the actual rendering. Frame chunking allows you to run multiple frames per task. For quick renders or for Nuke comps, this can be much more efficient as there is some overhead in the startup and shutdown of an instance. Also instances have a 10 minute minimum usage.
In the submitter UI you have a "Frame Chunk Size" which defaults to 1. In the example image, setting the chunk size to 5 would cause this submission to have 40 tasks, with each rendering 5 frames.