Shared Lists provide you with the ability to save a list for reuse over and over again.
You have the option when creating a list to 'Save to Your Items'.
When this is clicked you need to give it a name for the reusable version of the list:
This will then be selectable when, instead of creating a new list, you choose to add a list from your items as follows:
Notes
1) Deleting a reusable list will NOT affect forms that have that list.
2) If you update a reusable list, the updated version will be available to select. It will NOT update forms that already had that reusable list on it.