The technique began in the Chinese underworld, and it amounts to an unusually protracted form of social engineering. The analogy is with fattening up a pig, then butchering it for all it’s worth. In this case the analogy is wayward, since the criminal doesn’t really fatten up the pig, not that much, anyway, but it works at least this far: they develop the marks slowly, and they get the marks to fatten up the accounts they ultimately drain.