There are many reasons one might choose to have therapy. Here are a few examples.
Grief that won't heal
Inability to realise your potential
Loss of meaning
Patterns of self sabotage
Blocks to self expression
In fact there are as many reasons to get therapy as there are people. But one thing is the same for everyone, whatever your issue happens to be, the answers are all within you. There is a subtler, deeper level to who you are, and that is where your healing and insight come from. Sometimes you lose touch with that inner source of healing. A therapist is someone who helps you reconnect.
Therapy works for a really odd reason, namely the fact that one does not always know what one knows. That is why a therapist is useful. A therapist helps you open up to yourself. This opening up will result in a shift, and that shift is what will heal you, free you, enlighten you.