A good example is the ageing of the societies with persistently declining shares of youngest cohorts. Moreover, there are frictions in education and labour mobility that can further diminish the productivity potential of younger generations. All these factors lower aggregate investment and demand for capital.
If they don’t reduce aggregate income and savings at the same time, they can ultimately lower the natural rate of interest. Why do these mechanisms matter for monetary policy? Because the lower the natural interestrate level is, the higher is the probability that nominal interest rates are near the effective lower bound – “hitting the floor”, if you like.