Ok so I take that as a no! 
Yet a good question - here are few reasons, quickly off the top of my head (but actually not the motivation for the OP) and I’d be sure happy for others to chime in if they see value.
Pros
- Entice users to signup and engage, extra feature/bonus - added value.
- Increase engagement on some sites (so you use it!)
- Preserve the function for serious community based on trust (in line with Discourse trust-thinking).
- *Reduce server load on big and busy forums. (I do not know if technically that is a thing!)
Cons
- It might reduce anon users engagement on some sites (so you don’t use it!)
Being able to simply toggle it on/off for logged in / logged out users might be just right for some sites - others might find benefit assigning it to trust based levels.
My motivation to ask was to see if it was a feature anywhere that I had missed based on user feedback, so following it up and I did try search the topic before I posted 
*a foggy memory of early days when you didn’t want the search function to be hit by bots or abused using up server resources but this may be utterly out-dated and irrelevant!