When working on plans at a high-level with Epics you can assign a Team you would like to work on the Epic. This is great, but if the Epic is large (because it hasn't been broken down yet), then it would be really useful to be able to tell Portfolio that you would like two or more teams to be able to share the work on this.
Say the Epic has a high-level estimate of 100 story points, the difference this feature would make is that having multiple teams working on it in Parallel the elapsed time would be shorter, rather than a single team working through it.
When the Epic gets broken into stories, then multiple teams could work on it anyway, so this feature again would help the allocation engine decide which teams could do this work.