Proper identification is necessary to check the further spread of the pest.
Moreover, to know the ETL (economic threshold level) or the economic injury level is necessary to understand the economic feasibility to control the pest.
If the cost of insecticide is more than the the total loss caused by an insect in a particular crop than it's better to avoid any chemical control measure. Better to go for cultural or bio-control measure which are relatively cheaper and environmental friendly.
you need it for three reasons:
1) to estimate potential yield loss;
2) to chose right agrotechnology (including crop rotation), pesticides or biocontrol agents to fight pest/pathogens/weeds
3) to chose right crops/resistant cultivars to reduce the economic losses.