General Steam exposure, legitimization as a dev/pub by padding release list, monetization via item drops, monetization via telemetry (if injected), monetization via cryptominer (if injected). There are many ways to gain from Steam by publishing a free game.
Please, get an attorney and have them file a DMCA (which proxies out your personal information from being communicated to the thief). People like this don't deserve to get away with their crimes.