I assume they’re trying to fight against bots buying up “hot” Airbnbs that sell out quickly. It’s a weird captcha for sure, but understandable why they wouldn’t want bots accessing the site.
There are also anon account crews that will make many fake accounts using sourced credentials from their social group or bought on darknets. Generally, those only result in things like pop-up events from which entry fees are collected. I assume some could use the same method to commit robbery. Another method is to use a botnet of fake accounts and use the bots to build up fake reputation, eg offer a fake listing, book the fake listing, leave good reviews, etc.
There are also some weird currency arbitrage opportunities that might be found if someone was really inclined. From what I saw of that aspect, you'd have to be really looking for pennies in the couch seams.
Source: Did major customer support work for Airbnb pre-ipo
It's a market, so it's potentially susceptible to HFT and arbitrage bots like anything else.
You want a place to stay, inform the bot "give me anything good in this area when available", pay the bot extra money, and then the bot goes out and hits the good rooms in the area the moment they become available.
Nobody ends up staying in them, but it can bankrupt the Airbnb host pretty quickly if they have many properties on highly leveraged mortgages.