Black tea extract packs the polyphenols of tea into a capsule, standardized for theaflavins and catechins. You get a much higher dose than you would from a brewed cup.
Many people take it to help maintain blood pressure and cholesterol levels already in a normal range. The extract also supplies antioxidants that counter oxidative stress.
It contains caffeine, so it can sharpen alertness and energy. Take it earlier in the day with food to avoid stomach upset. Skip it or choose a decaf version if caffeine bothers you.