See, Twitter doesn't display every tweet of yours to all your followers. Anything that begins with an "@" is treated like a reply, and has a pretty limited reach. You'll see @-beginning tweets only if both the person tagged and the person tweeting are people you follow. In recent years, Twitter has started to separate out "reply" tweets more and more. When several tweets written as replies to previous ones become a longer conversation, Twitter bundles it for you and shows it to you as a linked string of tweets.
But not every tweet that begins with an "@" is actually a reply to another user. Sometimes the tweet is simply a sentence that begins with a reference to another user. Other times, it's a stage whisper: The tweet is addressed to one person, but it's meant for everyone to see. To get around the fact that Twitter automatically limits the reach of and "@"-leading tweets, even if they're not actually meant as a reply to a previous tweet, many users put a single character, usually an "." right at the beginning, to prevent Twitter from handling it like a reply. Celebrities do this a lot.
But ".@" no more! Twitter will now automatically show those stage whisper tweets to all your followers - so long as it's not tweeted in reply to something else. Those actual replies will remain limited in reach, in the same way they are now. But Twitter has a way to make sure you can force all your followers to see your witty comeback, should you wish to do that.
Introducing the self-retweet, or a new thing Twitter is allowing its users to do to draw attention to their own tweets and replies. Once Twitter enables the retweet function for all users to use on their own tweets, Twitter says, "you can easily Retweet or Quote Tweet yourself when you want to share a new reflection or feel like a really good one went unnoticed," Twitter's announcement explains.
Like that ".@" elimination, the self-retweet isn't the result of a totally new invention. It's already possible to place a link to your own tweet in a new one. Some users have taken to replying to their own tweets over the course of many days to revive the entire thread for their followers as a bundled conversation. And there's always the manual retweet, although these days that's considered to be a very bad thing to do on Twitter. The self-retweet change simply makes it easier to do something its users were doing already.
Will hard-core users still ".@" for old time's sake? Probably, for a little bit. But longtime users are used to adapting to Twitter's attempts to absorb the organic, creative workarounds to the platform's limitations in the past.
Twitter didn't always allow users to post photos, so someone made TwitPic. Twitter then made the service irrelevant in 2011, when it announced a new capacity to host photo and video itself. Twitter didn't always automatically shorten URLs, so users used link shorteners to minimize the number of characters a link ate up. Now, Twitter won't count media attachments towards the character limit.
Since the changes haven't even rolled out yet, it's too early to bury the ".@" next to the manual retweet in the graveyard of obsolete Twitter conventions. But it's not long for this world.