Supervision is a professional relationship of looking along ‘over your shoulder’, yet it’s also one of extra support.
Thinking along about what you think a client needs means extra care for that client. Two know more that óne. Supervision is also crucial for discussing ethical issues.
At the institute where I was trained (Metanoia Institute in London) it is custom to have supervision not only during your training, but during your entire career. Also for really seasoned therapists input from colleages is of great value.