You get or used to get true believers working in hellish conditions[1] on the boats, paid ~nothing. It might be a quid pro quo convenience for the Tom Cruises, but there are also some suckers.

[1]: https://en.wikipedia.org/wiki/Sea_Org#Lawsuits

Having a few famous Tom Cruises claiming it helped them is almost certainly part of the strategy for hooking in the dubious.

Cruise's contribution is influence due to his status and likely money given his income. Other's contribution is going to be much simpler, as you pointed out.