1. If you assume the population increases (either naturally due to increase in births over deaths or through immigration exceeding emigration), economy has to expand to provide same standard of living. It is like if you have an extra child in the family, you need to earn more to keep up your standard of living.
2. There is a general expectation that as time progresses, everyone should live a little better. This is supposed to be “progress”. Some of this expectation is based on improvements in science & technology. But the notion that next generation should live better than us is an ancient human desire. This requires that economy expand to provide more resources for same number of people, even if population remains constant.
Now if you multiply 1 and 2, you can see how an expanding economy is perpetually needed if you don’t want to feel like you are running in place (or even falling behind).
