Once a hosted feature layer has been created, you are able to create a domain for a field but you can only add one code to this list at a time. This is a time consuming process for some lists and ideally you could use a table or perhaps even an existing list from another domain on AGOL to quickly fill this list.
An example where this would have been useful is I have a list of tree species common names on one of my hosted feature layers that I had created before. Recently, I had to create another hosted feature layer which also led to the need for having this same list after the hosted feature layer had been created which means my only solution at the moment is to create this same list one code at a time on AGOL.
