A ticket from Boundary Zone X is an extension to a Travelcard, the equivalent of an excess fare. There is no requirement for the train to call at any station in any Zone. Boundary Zone X tickets are often cheaper than tickets from named stations, but not always. The exact rules on routeing is a matter for debate but in this case, I think it's fair to say that any interpretation would ensure the ticket is valid to Southend Stations or Southend Victoria. If you buy a ticket from Harold Wood to Southend Stations, the price is the same as the Boundary Zone 6 ticket, yet you are restricted to trains at call at Harold Wood and then only on the NXEA route. So it's a "no brainer" to get the Boundary Zone 6 ticket on that line! On c2c the benefits are less, because c2c have lower-priced Super Off Peak tickets available on their route.