Correct.
I will add some more points to your reply:
For some natural calamities where Electric wires, poles, etc gets damaged, to run Freight, passenger trains which gets stuck at certain stations during that times, Diesel locos are used to pull the stranded rakes.
Also, during some major accidents, where Electric poles and wires gets damaged, to pull the undamaged portion...
more... of the rake of the train which is involved in accident is pulled to nearby stations by diesel locos.
Also disel locos are used for shunting trains in yards/ coaching depots.
For many zones in New lines where electrification work still not complete also for unelectrified lines diesel locos r used.
There is some more reasons apart from this. So, i will say adding My points and Mr. Ahin_22 points together, there are many reasons why every zone needs Diesel locos to be kept and to be used.