You don't. Just like you don't need light bulbs. And yet, most of us choose to use them. BT provides you with two valuable insights--identity bifurcation, and metacognition. What do those mean?
First, is bifurcation. Defined as the separating into two parts. BT offers the bifurcation of who you are--via nature and nurture. Once you understand your type (inborn), then you can make sense of your past/upbringing and understand the whole of who you are, and who you've become (via nurture). No other personal assessment method (e.g. Enneagram, DISC, Myers-Briggs, Big Five, etc.) offers that insight.
Second, is metacognition. Defined as one's awareness of and ability to regulate one's own thinking. BT enables you to see outside of yourself, and understand (objectively) how you think. From there, you can navigate life in a more circumspect and balanced way.
BT is also has heuristic value, which is another big word. Heuristic means enabling a person to discover or learn something for themselves. BT allows you to explore your past, in learning to understand your present.
To learn more about the letters and what they mean, please go here.