The motive for 'The Conspiracy' is unknown (unlike the existence of 'The Conspiracy', no specific motive necessarily follows from the FET) and as such it is open to speculation. However, financial profit is the most commonly assumed motive.
Usually considered to be the most likely motive, this suggests that NASA is primarily an embezzlement front. The conspiracy spends millions of dollars in bribing astronauts, faking photographs, and other ways of faking space exploration, but NASA's budget is in the billions, so the conspirators still receive a large profit yearly.
'Not hiding anything at all'
It is also possible that NASA does not actually know that the earth is flat and since its inception has simply been faking the concept of space travel, never bothering (or unable) to go any farther than the edge of the atmosphere. The earth is portrayed as round in NASA media because the general public already believes that it is round.
NASA is mistaken about the earth's shape, just like many others are, and reflect that mistake when putting together its fake space missions.
The US Government and its European allies have a large interest in investing untold millions of dollars into hoaxing space travel because it gives a superior image to the rest of the world. Once a country has the technology to reach orbit it can also scare off aggressors and send ICBMs raining down on its enemies at the push of a button. Creating the illusion of space travel is critical to national security of a first-world nation, whether it is actually possible or not.