Just imagine you rent an apartment for just you, your partner and your child. As time passes, you might notice how your family begins to grow. There might be more children coming their way. So, more rooms will be needed.
Likewise, it is not just the number of children you might have. Perhaps you only have two children, but they also grow up. And once they begin growing, they will need more space for their stuff: a pair of soccer cleats, a bag of balls, shoes and so on. Everyone starts gathering each own stuff. And one day, you will realize that there is not enough space for them.
This is a good reason to start looking for a bigger place, where everybody can have their own space to enjoy and grow.