the transmission of electrical energy have some losses. if 
we step down directly from 33kv to 415v. the resulting 
voltage may only an under voltage. in order to avoid that 
we have some restrictions.

It is not advisable to step down directly for following 
reasons.
a) If insulation between 33kV and 415V winding fails, then 
before triping, for a fraction of second 33kV high voltage 
will get connected to 415V system, on which vast users & 
equipments are connected.
b) Fault level on 33 kV is more, hence if fault occures on 
415V bus side, heavy kA fault current will flow as compared 
to 11kV/415V transformer.


But nowadays, 33kV/415V transformers are widely used to 
avoid double transformers

if we conside 33kv/11kv &11kv/450v the energy transfer is 
same as by 33kv/450v so current level is same for both

11kv is basically for primary distribution say to a large 
factory. so at a substation, stepping down to 11kv for such 
use and further stepping down to 415V for residential areas.