Ah, okay. So I don't benefit from enslaving them.
You benefit from them having lesser blocks, affection losses, and protections from actions as listed here in the wiki:
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You also benefit from them having a lower salary cap (or at least that was the intended purpose). This is meant to balance having 0 salary combat monsters, since the more battles you bring them to, the higher their fame. The higher their fame, the more recognizable they are to the general public, conferring a certain air of status to them, allowing them to ask for certain things, not unlike gladiators of old.
If you play the default, lore correct setting without mods, slaves never require any salary.
The lore correct setting according to Grimdark is a Rise from the Ashes start on Grimdark Difficulty A Dying World, Ironman mode on, and salary demands on. Your champion gladiator or prized fighter just wants something for his services now that he's made it big. Even historical Roman gladiators get certain favors and prizes. He gets prize winnings (the top ones outearn a legionary), product endorsements, merchandising, a girl, and something to drink. Your combat slave with 1000 fame is basically a war hero, known throughout Ikaanos. As a slave, he cannot just break contract. A servant or mercenary will break contract and just leave you without you being able to stop it in the conclude day screen. A slave can be stopped. But he can sulk and be angry. You can abuse your workshop worker, but not your champions.
