It drives active users and increases activity on the site. Reddit tracks site usage metrics, and active user count + engagement are two of the most important metrics, since more active users = more eyeballs on ads, and more engagement = more ads that can be placed in front of those eyeballs.
The fact that the majority of the new active users are bot accounts that can’t be advertised to is secondary, since the people who would invest in a reddit IPO wouldn’t typically look that deep, they’d just look at the top line metrics and go “oh, there’s a big bump on activity, this is a healthy website.”