A voyage that circles the globe over several months is sold both entire and in pieces. The segments are not a concession to demand but the structure that makes the voyage viable.
Few passengers can take the whole thing
A full circumnavigation requires a block of time that very few working travelers have, so the market for the complete voyage is small and largely retired.
That market alone would not fill a ship, and an underfilled vessel on a long deployment is expensive to run.
Selling shorter legs opens the same voyage to travelers who can take two or three weeks, which is a far larger pool.
Segments are defined by air access
A leg has to begin and end where passengers can realistically fly in and out, which means a port with an airport of adequate reach nearby.
This is why segment boundaries fall at particular cities rather than at even intervals, and why some attractive regions have no segment break.
Immigration and customs capability matters as much as flights, since a full passenger exchange requires clearance facilities the port must be equipped for.
Turnover changes the onboard experience
At each segment break a significant share of passengers leaves and a similar number arrives, which resets the social composition of the ship.
Full-voyage passengers therefore experience several distinct ship communities across the trip rather than one.
Lines respond with programs and recognition aimed at the continuing passengers, since they are the ones present for every reset.
Pricing works differently on each
The full voyage is generally priced below the sum of its segments, reflecting the certainty of a cabin occupied for the entire deployment.
Segments are priced against what a comparable standalone cruise in that region would fetch, so their per-day cost varies widely across the voyage.
Segments crossing long stretches of open ocean tend to price lower, for the same reason repositioning voyages do.
What to weigh when choosing a leg
The flights at each end are usually the largest variable cost, and a segment ending at a poorly connected port can cost more to leave than the leg itself.
Visa requirements accumulate across a long leg, since a single segment may touch several countries with different rules for arrivals by sea.
Sea day proportion is the other thing to check, since two segments of identical length can differ enormously in how many ports they actually visit.