MBCT has a narrower purpose than the word "mindfulness" suggests, and understanding that purpose is the key. It was built by Segal, Williams and Teasdale for a specific problem: relapse into depression.
The data behind it are sobering. After a first depressive episode, the risk of a second is roughly 50%. After the second, the risk rises towards 70%; after the third, towards 90%. Something becomes automated.
What becomes automated is rumination. In someone who has been through a depression, ordinary sadness reactivates a whole pattern of thinking — "here it comes again", "what's wrong with me" — that turns a passing mood into a full episode. MBCT trains precisely the capacity to notice that reactivation without entering it.