I'm a layman so by my limited understanding..
reason 1 to buy time!
to calibrate deep space network etc. as prasanna said.
its india's first launch into deep space..
we're toddlers in comparison Nasa's deep space network Goldstone-Madrid-Canberra, where NASA is ahead by many years..
There is also coordination with exterior Sites during the early phases:
.. so with hardly any 'experience' in this its good to take things slowly. the stakes are high..
Reason 2 Extented orbits also gives the opportunity to plan and simulate orbital transfer strategies for future, possibly interplanetary missions. As I am sure India will not have the extravagant budget of NASA to conduct seperate launches to test for all the steps leading to more complicated missions.
and to avoid babbling about stuff i don't know too much about
: here's the official orbit plan and diagram: