I think it would be hugely helpful if users can change the creation date. Because that's where multiple sorting options come into play. If you're bringing in dozens of posts from another platform, yes, you can make a custom date field, but it would be much better to be able to modify the actual creation date. Then entries can be sorted in a variety of ways by that date.
The way it is now, you can create a custom date field, but all the sorting is done by the actual creation, update, or publish date, which means if you bring in old posts out of order, they won't appear to be sorted correctly if you aren't careful about the order you bring them in.
Does that make sense?