extranonce.subscribe

I just received an email from a user requesting that we add support for extranonce.subscribe; however, after researching, I cannot find out what the purpose of this feature is. Does someone know what the feature actually does? Thanks for your help in advance.

This is the best i could come up with,
https://bitcointalk.org/index.php?topic=9438.msg136344#msg136344

I looked at that.

There's plenty of information out there on how to implement the feature, and it's not difficult to add support for the subscription to extranonces. But I can't figure out the reason why someone would want this feature. In particular, there is no documentation on when the pool should send the set_extranonce messages.

I'm going to keep searching for a few more hours, but I'm not going to waste time implementing a feature if I can't understand the reason why it would be needed.