In OLTP environments using traditional RDBMS's it's very common to periodically rebuild indexes for both performance and space savings. I am just guessing that mongoDB would need the same option. reIndex() works perfect, however, it's not enabled for background yet. I would contend it's most important for reIndex() because of the typical rollout of projects (create new indexes, run for a while, oh, we need to reindex but we are live..).