To a large degree, this is like the stupidest question ever.
However, if I Update a field in the current list item, let’s say a date/time field, can I also just Set the same field in the current item? Yes and No.
The operation of Update, which is an instance of the UpdateItemActivity class in SharePoint.WorkflowActions, is obvious enough. On the other-hand, it is not unreasonable to think that Set would behave the same way – except it does not. Set takes effect if and only if the field is not set, i.e. unused at the time the Set executes.
This is fine. Undocumented as far as I can find, but fine nevertheless. When you want a field to be set once, and then become read-only, use Set. Otherwise, if you really want the Update behavior, use that. It’s that simple, and so simple, it should have been documented!!!