AFIK when you send a book, Kobo goes through the processing screen when you disconnect. This makes changes to the area in the database the book info is stored.
When you reconnect with out sending that book, things like series info and book location can be stored without being overwritten.
In my experience resending a book does not always overwrite reading location although sometimes it does.
I believe that is why it is done in two steps. Better safe than sorry
Helen