It's probably the cheapest way to raise awareness. If each can draw 200-1000 viewers (and some more) that's more eyes on than YT ads, just as well-targeted, and likely at cheaper rates. Debuts are also that one exception where vtubers naturally get excited about content cross platforms and cross agencies, and being paid to do react content is a Twitch streamer's dream job. I wouldn't be surprised if the rates were low just because debut react is something a vtuber might do anyway and it has the 'free content' factor.
Awareness doesn't equal viewers, but it certainly helps and it's one of the reasons small corpos sometimes languish in obscurity. It's like subcount, it's not a direct translation to views but it can contribute.
edit: Also, I think Aviel is still in the "try stuff and see what sticks" phase. He's tried ads, now he's trying paid watchalongs. If this doesn't pay off, he'll know next gen.