This button opens an entity source selection and then copies attributes with the same name automatically into the current create / change entity action.
So if you're for example looping through a list and you want to copy the values in the iterator entity into a newly created entity this could save a lot of time.
And it helps with normalizing data quickly.
Confirm with an "Are you sure?" and overwrite the initial values set.
When cloning the attributes by name, ignore case.
Ignore any attributes that do not exist with a similar name.
Real magic would be to automatically do conversion from string to date etc too.