Promoted tweets, soon in a thread near you

Promoted Tweets Set To Appear in Your Twitter Timeline

“Now, Promoted Tweets will show up directly in the stream, with one caveat: You’ll only see ads from accounts that you already follow.”

But if that is so, why would companies use Twitter ads? Why not use free regular tweets instead? What am I missing?

“Twitter also says that the tweets can be “easily dismissed from your timeline with a single click.””