@mentions are a way for you to engage other Twitter users in your tweet. If you’re posting a tweet that you think is relevant to specific followers or thought leaders, tag them at the end of your tweet with a @mention as this will call their attention to your tweet and possibly the attention of their followers.
For example, Zack Wenger put together this video that featured Mark Cuban. Wenger only has 1,000 followers, but by mentioning @mcuban in his tweet, he brought it to Cuban’s attention. Cuban retweeted it, making his own 4.3m followers aware of it, not just Wenger’s 1,000.