When you stay close and conscious of your immediate experience, you aren’t lost in thought. The mind generally likes to take present experience and elaborate on it (mental proliferation). Mindfulness reduces and even eliminates this tendency, leaving a mind quiet and simple. This reduces anxiety and speculation.
The mind likes speculation, thinks it’s a worthwhile venture. It thinks it’s fulfilling its mission. Mindfulness has a different slant on that. Its goal is direct experience, not thinking about but seeing directly. That’s the source of direct vision of Truth, and higher reality. That’s the Miracle of Mindfulness!