The importance of population to a country is well understood in countries where the birth rate is falling and the number of working people is decreasing. Then those countries have to bring in skilled manpower from other developing countries to keep their country running. In the coming days, countries with high skilled youth population will be able to benefit more economically, as countries with low youth population will have no option but to hire skilled manpower from other countries.
Therefore, all countries will want to hire skilled people in the employment sector, as a result of which the skilled people of a country will make that country important to other countries in various ways. So the population of a country affects many things including the economy of that country.
exactly you can only realize the benefit when you don't have something but if you possess all the things then you cannot realize its advantage same is the case with population that if a country's population is increasing then they consider it as unfavourable but if they don't have much population then they desire to have more people to work for brightening the future of a country.
In some countries where there are lots of people have no extra seats for job and unemployment increases therefore individuals from such countries go towards others countries to earn passive income to make their life.
I realized and have experience that a country will be powerful if it has consumate people with creative mind but its also true that if a country possess larger population but all of them are lazy then such larger population will have no significant for country.