One of the things oft ignored in science fiction, is that once such a ship leaves on its journey, the cost of doing so becomes nil.
That is, the economy on the ship is entirely self-contained. Talking about 'earth dollars', 'earth salary', and 'earth living costs' is meaningless. There ARE no salaries paid from earth. There ARE no support costs from earth. The inhabitants of the spaceship are as rich as they ever will be, in earth dollars. That is, absolutely penniless. But in terms of PPP, they would be as rich as the collective manufacturing capability of the spaceship. They could never own more than can be produced (or originally supplied) on the ship.
The fact that they would never be PAID again is, of course, moot, because there is nothing for them to SPEND their income on. They can't send it back to earth.
So one benefit, and a not insignificant one, is that these inhabitants, and their descendants, would be living 'free' in any earth-based sense. They, and their descendents, would never have to pay rent, a mortgage, food, car loans, or any other expense ever again.
The potential to live in a perfect socialist commune type community, forever free from the yoke of money and of conceding it to others, is a very powerful motivator for many.
In a sense. it would be the perfect 'convent', the perfect 'cluster'. And it pretty much guarantees that all of your descendants would live by the same philosophy.
It would be very appealing to a distinct ideological group who wished to enshrine their belief system in an environment that ensured it could not be altered, immune from any social or economic pressures from any other ideological factions on earth.
Incidentally, there are a good selection of such ideological groupings on earth that are sufficiently wealthy enough to pull off such a feat.