Tweets that you read and accounts that you follow are fairly likely to be on subjects that you care about. Tweets that you write are almost guaranteed to be about something that matters to you.
Twitter says this is important for advertisers, since it lets marketers reach the right audience at the right time, in the right context.
And they’re right, to a degree.
Twitter’s optimistic scenario is a user tweeting about a great song from their favorite band. As it happens in contrived examples, this band just happens to be playing a nearby venue that very night, which means that promoters can run geotargeted Twitter ad campaigns against local users who have tweeted that band’s name, and thereby inform this particular user that her favorite group is in town.

There’s no doubt that there’s huge value in this new targeting option. And, as AllThingsD notes, Twitter has included negative sentiment filtering, which is critical, because no advertiser wants to target people who are tweeting about Wu-Tang Clan, only to find that they were tweeting that Wu-Tang Clan sucks, or something similar.
But it’s also challenging, because many tweets are difficult to register commercial intent for.
If I tweet about my wife’s illness, are you going to target me with a random medicine? Or if you tweet about a great dinner you’re just about to eat, will you really be receptive to ads about a Greek restaurant just down the road? Twitter says it’s helping advertisers target “signals of intent,” and that might sound like Google-ish search keyword targeting, but it’s not clear that the link is quite as obvious as an intentional, directed search.
But it is yet another tool in the Twitter advertiser toolbox, and with all the others, will add up to better results. Twitter says that it has tested the feature with Walgreens and Microsoft, among others, and “users were significantly more likely to engage with Promoted Tweets using keyword targeting in timeline than other forms of targeting in the timeline.”
Which sounds like Twitter ad products are synergistic: 1 + 1 = 3, not 2. And in fact, GoPro achieve engagement rates as high as 11 percent, Twitter says, using the new feature. That’s startlingly high.
And, doubtless, music to advertisers ears.