Growth matters both for fiscal stabilization and for raising living standards
Broadly speaking, there are two main sources of economic growth: growth in the size of the workforce and growth in the productivity (output per hour worked) of that workforce.
For a given labor participation ratio, increasing population means increasing the size of the workforce.
For a given productivity, increasing the size of the workforce means increasing the economic growth.
GDP measures the market value of goods and services produced in the country, but it captures only market activity and is not designed to be a measure of economic welfare. A parent in the paid labor force contributes to GDP; one who stays home to take care of children or an aging family member does not, but, if the family hires someone to perform these same duties, that labor would contribute to GDP. Health, safety, and environmental regulations can impose costs on businesses that may slow measured GDP growth, but any such costs must be compared with the benefits of better health, safer workplaces, and a cleaner environment that may not be captured in GDP.
Some flaws of using GDP to measure growth, haven’t found a better way to measure growth.