Erlang has the ability to upgrade the code of a live system. This is a feature which is very appreciated by our customers. They can apply the patch we provide for their Instant Messaging system without having to stop / restart the service. This article from Bill Clementson does a good job in explaining what is the magic behind Erlang live code upgrade.
For production systems gathering hundreds of thousands of users online having the ability to apply a patch on a system without having to restart it is a major feature. It allows them to offer a high-availability service.
In real deployed system, this is the type of feature that makes a difference. Our secret for ejabberd live code upgrade feature relies on Erlang. Bill Clementson has made a good job summarizing how it works on his blog: Live Code Update in Erlang.
After reading this article, this should now seems obvious for every one and a must-have feature for production systems.