Entering the population size by age group enables you to estimate the per capita flu burden (per 100k population). It also enables the option to project the impact of vaccination campaigns on the estimated flu burden.
I know the population size by age group for at least one season.
Normalizing the flu burden by population (or sub-population) size—i.e., to generate per capita values (per 100k population)—enables direct comparison between countries, groups, or flu seasons with different population sizes.
Population (or sub-population) size is needed to link the flu burden estimates generated by this tool to the World Health Organization (WHO) tool that estimates flu burden averted through vaccination. The WHO burden averted tool estimates the direct impact of flu vaccination campaigns on averting flu infections, medically-attended cases, hospitalizations, and deaths. It helps users to explore how changes in vaccination coverage and timing of vaccine deployment can affect the impact of the vaccine. The WHO burden averted tool and its documentation can be found here: https://www.who.int/publications/i/item/B09385