As population expands,there is more people to work.Which bring more job opportunity in form of people intensive or outsourced projects.These develop the the skill set of the population.They in turn start own venture bring in new ideas which provide more employment. This vicious cycle bring in economic development.This is what exactly happened in India .
Yes, population expansion helps nations, because there are more people producing so that they can pay taxes. Population expansion also generally just makes a nation more powerful. More people need to buy goods, so they are more likely to be an economic player. Population expansion also makes an army larger.
Populations expansion requires more resources and military force from nations. In some cases it helps because they nation can increase the amount of resources it has, but it depends on the ability of the people and what is in the areas that they have. It depends on the circumstances to determine if it helps or not.
I do not think that population expansion helps nations. I think that a lot of nations are already having a difficult time taking care of the people that they have in their countries. And even the richer nations are having to deal with an issue with their economy due to an expanding economy.
I do not believe population expansion helps nations. This certainly is not the case in every nation, because we have many in Africa that experience population booms without any significant change to their society. We have also seen, in history, instances where population decline has actually been helpful. When the plagues raced across Europe, they actually ended up helping the people who survived and helping the nations function better in the years after wards.