Cruise lines sell air travel alongside the voyage, usually at a price a traveler could beat online. The difference is not the fare but what happens when something goes wrong.
The line becomes responsible for delivery
When the operator books the flights, getting the passenger to the ship becomes the operator's problem rather than the passenger's.
A missed connection triggers rebooking by the line, and where that fails, transport to a subsequent port so the passenger can join the voyage in progress.
That obligation is the product. The seat itself is an ordinary airline seat bought through a contracted channel.
Fares are contracted rather than published
Lines hold negotiated fares with carriers, which sometimes undercut the public price on long haul routes and often do not on domestic ones.
Those fares frequently carry restrictions, including limited schedule choice, no seat selection until close to departure and reduced or absent frequent flyer accrual.
Deviation requests, which let a passenger arrive early or stay later, are usually available for a fee and are the most common adjustment travelers make.
Ticketing happens later than expected
Cruise air is often ticketed well after booking, sometimes close to final payment, and the specific flights may not be confirmed until then.
Travelers who need a known itinerary for other arrangements find this uncomfortable, and it is the main reason experienced cruisers book their own air.
It also means the routing can be one the passenger would not have chosen, including connections through hubs with tight timings.
Independent booking shifts the risk
Booking flights directly gives full control of carrier, routing and timing, and allows the use of miles or a preferred airline's benefits.
It also places the entire consequence of a delay on the traveler, to be recovered, if at all, through insurance rather than through the cruise line.
The cost of catching a ship at the next port is the exposure being taken on, and it varies enormously by itinerary.
How to decide between them
The comparison worth making is the fare difference against the realistic cost of a missed embarkation on that specific voyage.
For a cruise leaving from a port a short drive away, the risk is small and independent booking is straightforward.
For a voyage starting overseas after a connection, arriving a day early independently removes most of the risk, and doing both is what most careful travelers end up choosing.