From SA Mafia Wiki
The RNG, or Random Number Generator, is a staple of mafia. Roles are assigned to players randomly through an RNG, without any bias about who gets what roles among the signups. This is to prevent any moderator bias from sneaking into the game, either through favoritism in giving your friends the best roles, or pettiness in giving your enemies the worst. The RNG principle is held sacrosanct by mafia players, such that it is assumed all games are assigned via RNG unless otherwise specified, even though it risks being unbalancing.
An RNG that most mafia players use when creating setups can be found here.