Can someone explain all this talk on thermal grease? I've read a lot of this thread, but it isn't entirely clear to me.

From what I understand, you are taking off the heat sinks, adding thermal grease, and then putting them back. Why do you have to do this? It seems it makes it cooler - does that make them hash faster? Which heat sinks do you replace? Is this a problem only with Batch 1-3 or something, or would you recommend this for all of them?