There are some instances where data in mongoDB is publicly open.
It would be extremely efficient if you could query mongoDB (only read-only operations) using built-in Simple Rest Interface
http://docs.mongodb.org/ecosystem/tools/http-interfaces/#simple-rest-interface
from web applications.
To do that the server must simply return
Access-Control-Allow-Origin: *
as additional header. It already returns custom headers so this new one should not be a problem.

The main reason why this is so useful is we have mongo databases distributed globally yet our web servers are located in one region.
When users access the web application from other regions JavaScript code could simply query the in-region mongoDB instance.

Obviously if we go with Sleepy Mongoose or any other outside of core system interfaces we have to send our requests back to the web servers.