So take an example of three jobs (submitted one after the other with equal priority), each with four tasks (say frames) and you have 2 available render machines:
Job 1: 01, 02, 03, 04
Job 2: 01, 02, 03, 04
Job 3: 01, 02, 03, 04

Renderer A takes Job1_task01
Renderer B takes Job1_task02

They both finish, and are available to receive another task each. We now have:
Job 1: 03, 04
Job 2: 01, 02, 03, 04
Job 3: 01, 02, 03, 04

Currently, the manager will send out the remaining two tasks from Job 1. I would rather it send out the first task each from Job 2 and Job 3. So the jobs finish at about the same time, rather than Job 1 finishing before Jobs 2 and 3 even begin.