Domains play an important role in node servers reliability, f.ex. see recent article http://engineering.spanishdict.com/blog/2013/12/20/the-4-keys-to-100-uptime-with-nodejs

Node tries it's best to correlate callbacks to keep them within the same domain (setTimeout, files, network, etc), but sometimes its only possible in application code, for example when different db queries and answers use a shared network connection, as in the case of mongo driver. This is where the driver should help node.

See also discussion in Mongoose - https://github.com/LearnBoost/mongoose/pull/1337