Why do many airlines have the same flight number with a plane change at a connecting city?

For example, Delta 817 flies EWR->ATL, then ATL->OAK on a different plane. It is obvious that it is a different plane because on July 20 2006 (according to Delta's web site flight status), the EWR->ATL segment arrived at ATL late at 7:07pm (scheduled 5:54pm), while the ATL->OAK segment departed from ATL on time at 6:45pm. Must have been annoying for those passengers who expected to be able to stay on flight 817 to continue on to OAK, only to be told that they missed their connection.

From a passenger standpoint, it just creates confusion, particularly with occasional travelers who assume that a "direct" flight does not involve a plane change. (Or someone who previously flew Southwest, where a single flight number does indeed mean that you do not have to change planes, even if it stops somewhere before reaching your final destination.)

Its a good question, i always wondered the same thing with the US Air flights from LGW which had the same flight number from LGW-PHL-LAS and LGW-CLT-MCO, even though they involved a plane change. At PHL this could mean insufficient time to change.
I put it down to the fact that Brits go to Vegas and Orlando as holiday destinations and a "direct" flight might encourage more bookings.

The reason that airlines do this is to move higher on the flight list when routings are displayed. This in turn increases sales for that airline. It is a dispicable practice, but legal and some airlines do it a lot more than others. They pick routes on which they are not competitive and create these "virtual" routes just for marketing purposes. It is particularly bad when the aircraft types are not compatible -- so you cannot have the same seat assignment for both legs of the same flight number!

Wouldn't travel agents (including web sites) be aware of that and ignore the fact that two segments have the same flight number and just rank the flight as if it were just another connecting flight?

On the other hand, does this make much difference, since most people sort options after a flight search by either price or schedule, not whether the flight has the same flight number through a connection?

Well, spank my monkey - I thought they did this either because they are incompetent idiots (few would disagree with this), or because they get a sadistic joy out of messing with us. I especially like the "through" flights AA has at DFW, where you don't have enough time to deboard, change terminals, and reboard before the next boarding call.

This is when your travel agent should explain the difference between "direct" and "nonstop". Direct means same flight number, nothing more.

Beware of airlines robbing you of your miles too.

For example, if you fly CO 51 FRA-EWR-IAH-EZE, you get 7,134 miles for your 27-hr trip, instead of 3,859 + 1,415 + 5,062 = 10,336 miles. [It's a 772 for FRA-EWR, then the same 762 for EWR-IAH-EZE.]

Or CO 7 SAT-IAH-NRT-GUM. You get 7,293 miles for the 21-hr trip, instead of 500 + 6,643 + 1,558 = 8,701 miles. [738/9 for SAT-IAH, 772 for IAH-NRT, and 738 for NRT-GUM.]

So is there any advantage to the passenger for a flight where multiple segments have the same flight number? Seems like you're still likely to have to make (and risk missing) a connection (with the usual disadvantages like longer total travel time, greater risk of checked baggage loss, etc.), and you'll likely get fewer frequent flyer miles. (Assume a non-Southwest airline, since Southwest uses the same flight number only for the same plane, and gives one frequent flyer point per one way, regardless of number of segments.)

None that I can think of.

Given that miles earned for "direct" flights are fewer than if the itinerary is split into separate sectors, I believe that occasionally it means that fewer 500-mile upgrade coupons can be required for some routes.

At the same time, it can make it more difficult to upgrade if both sectors aren't cleared.