This is a question I'm asked frequently, when I'm running introductory courses or speaking to prospective students. The answer for me is that it is a beautiful theory that really works.
TA was originally designed to be accessible to all. The key concepts are described in everyday language and the basic theories are deceptively simple. You can pick up the fundamentals of a theory of personality or interpersonal interactions in 5 or 10 minutes. More importantly, you would also be able to put that concept into practice straight away. The concepts are so easily applicable that we can start to apply them to ourselves, use them to make sense of our day-to-day situations, and make positive changes immediately. Whenever I teach a TA101 -- our 2-day course of the key concepts -- I look forward to the second morning, as I can guarantee that some of the participants will come in describing how they have tried out some of the theory and seen its positive effects.
On the other hand, after 15 years of studying TA, I am still struck by the subtleties and possibilities of the theory. Those deceptively simple concepts, such as a model of personality comprising events in our own history (the Child), our experience of influential others (the Parent) and our being in the here-and-now (the Adult), are able to describe and inform issues as complex as trauma and personality disorders, and guide our clinical practice with clients. The theory of the Script describes how we adapt ourselves to others in relationship from the earliest age, and because of that we limit ourselves and develop troublesome patterns of behaviour. This has effects on our thinking and how we make sense of some situations, which can lead to problems such as panic attacks. It also affects our relationships, and the TA theory of psychological Games gives us a way to think about the problematic repeating patterns that we get into with other people: the roles we play and the ways we might sabotage ourselves. We can use that theory not only to help our communications and relationships, but also to understand the internal world of our clients in psychotherapy.
The purpose of TA psychotherapy is to allow clients to be released from those constraints of their own history. So they have more freedom in themselves: to develop different ways of communicating, different ways of seeing and thinking about the world, and different, more fulfilling ways of being in relationship with others.