When the population experiences growth, it increases the number of people who can consume goods and services, causing the economy to experience growth. When the economy grows, this results in new industries that can provide employment to the nation's citizens.
Another reason that population growth is beneficial is that it causes more people to exist who may potentially be great innovators or inventors. A larger population encourages its citizens to find new technologies to keep the ever-expanding numbers healthy and happy.
Some countries, such as China, are experiencing marked differences in the ratio of male to female citizens. Men who want to marry and start a family cannot do so because there are so many more men than women in these nations. Allowing the population to grow helps correct this imbalance.