In Jira Cloud, during a transition, all post-functions run in parallel. Often, however, you need actions to be executed in sequential order. This new post-function, Sequence of Post-functions, allows you to combine several JMWE post-functions into one sequence so that they can all run in a predefined order. For example, if you need, during a transition, to first create a new issue and then send a notification email mentioning the new issue, you need to make sure that the first post-function has finished running before starting the second one. The sequence would ensure that it runs properly.
Plus, it is much easier to set up. Just add this post-function to a transition, and then you'll be able to add and reorganize other JMWE post-function right inside it. You even have an option to stop execution if one of the post-functions fails.