We use this feature to keep track of projects that involve different IT staff. I open a parent work order at the beginning of a project. I then create assignments for different phases or needs of the project.
Parent work order: Replace Server
Assignment 1: Purchase server
Assignment 2: Test server
Assignment 3: Deploy
We keep track of hours in the assignments, and the parent work order sums the totals of the hours.
We use it in slightly different way: the team gets requests to set up events for sale. Because they usually run for some time, there often are changes: new prices, deals, discounts etc. to set up after the original event went live. All these additional requests go as assignments, so the whole history is clearly visible.