For purely remote scenarios, it is an important indication of whether you will be back soon or if you are afk for an extended period of time. Without it you are left broadcasting your status ("going afk for an hour to run an errand, etc") on channels, or having to hack up your profile each time and assume that other users will know where to look to determine when you will be back.
Just about every messaging system has support for this: ICQ, MSN Messenger, Jabber, Yahoo Messenger, Google Talk and now Google Hangouts, HipChat, etc. Even with IRC there are cheap ways to simulate it. It's really hard to get used to not having it after making good use of it for almost 2 decades.
I think the "2 decades" thing is relevant here. Most of the products you mention were created before people were almost always physically in contact with an internet-enabled device.
I never used any of those much, but starting using Slack when I joined a company where it was ubiquitous. I never saw anybody care about declaring an away message. People at any point could be busy and might not answer, but they were rarely "away". The culture was just to assume that people were available, and if not that they'd reply when they could.
I work for a slack competitor that has away messages, and I'm jealous of Slack's willingness to ignore the old interface patterns and implement the simplest solution possible that makes the most sense for larger group users, new users, ignoring the expectations of the smaller group of users, old-time users.
Just about every messaging system has support for this: ICQ, MSN Messenger, Jabber, Yahoo Messenger, Google Talk and now Google Hangouts, HipChat, etc. Even with IRC there are cheap ways to simulate it. It's really hard to get used to not having it after making good use of it for almost 2 decades.