As technology has developed in recent years, most jobs now involve sitting at desks. Generally people are less active than they used to be, and leading relatively sedentary lives compared to previous generations. This has affected human evolution. As a species we require less leverage from our limbs, and as a result human legs are getting shorter. The average height is increasing however, which means body proportions are changing. Furniture also needs to change to adapt to the new average body proportions, hence the need for modern benches. There’s an interesting article about this, which highlights the dangers of using old benches :
www.bbc.co.uk/human evolution/125877589