Why is population data important to a country?

It alerts the government to changes needed in the country.

e.g. If a census indicates an increase in the number of elderly people and a decrease in the number of young people then the government knows to cut resources to the education sector for the youth and spend the money on help for the elderly such as more care.

As well as being able to adapt to its current population the country might need to advertise migrant workers to come to the country. In the example I used you will see that both an increase in elderly and decrease in youth will raise the dependency ratio meaning there are less people of working age. This calls for workers from abroad to come and fill these spaces to keep the country running.

Population data is also required to monitor public transport and view whether a higher budget is required based on how many people use it etc.

In ELDC's (Economically Less Developed Countries) less can be done with census data as it tends to be very inaccurate due to many factors.

Terrain- census' cant always be delivered to remote places

addresses- not everyone has fixed addresses such as nomads who constantly travel

Tribal languages- The government often struggle to communicate with tribal folk who don't speak the first language of the country.

trust- Many rural people lie as they don't want the government to hold information about them

lies- Village leaders will lie in order to get more resources out of the government system.

Hopefully this answers your question.