Why do i need a diagnosis of autiesm?

Why do i need a diagnosis of autiesm?
signs:
- abnormal behavior, this may be a sign of physical or mental abnormality
  difficulties to relate with others; they isolate themselves; group we see the other missing. Often their eyes are fleeing.
It is practically impossible to engage in spontaneous play.
They speak badly or not at all, otherwise we understand and interpret what is said or what happens to them.
They are opposed to any form of change. They need a stable living environment: people, places, furniture schedules.