Dilatation will be clinical perspective  on examination finding by an Ophthalmologist OCT posterior segment is a non invasive diagnostic imaging which will give findings on exactly what and where is the problem. Both the above has to correlate each other. Only on clinical examination if the ophthalmologist( eye surgeon)has observed 1)any changes in retina or 2)If visual disturbance is correlating to changes in retina. 3)Incase of Cataract, retina view on dilatation clinical view will be hazy ,because of Cataract, In that case also there are few diagnostic tests  ( B scan , etc)with which we can find if any changes in retina are there. To confirm,to find why,and what and where exactly is the problem , how exactly to manage the further treatment options, diagnostic tests are further done.

Even if there's an early swelling in the macula, it will be associated with a reduction in vision. Sometimes OCT is required to detect subtle swelling in the macula. An Ophthalmologist will always correlate Vision and clinical examination to decide on whether OCT is required or not.