When your workflow is turned off, any incoming webhook data sent to the offline workflow is still saved. When you turn on your workflow, be prompted with being able to run executions that were missed while the workflow was off.
This would be very useful for when data needs to be synced between two platforms. It would ensure data validity in the second platform (i.e. no data is missing). It is otherwise a pain to find the missing pieces of data while the workflow turned off and having to transfer them manually.
Make (Integromat) does this quite well already.