Is there a reason to get final diagnoses of autism?

My son is 3 yrs old and his neurologist and peditrician thinks he is somewhere on the autism spectrum and prob is pdd-nos.They refered me to a child psychologist to get diagnoses.My son already take OC,PT and speech for his sensory issues and severe speech apraxia.He is also in a special ed prek at school.Is there really any medical reason I need to get a diagnoses of autism or since he gets help anyway could it wait.