It is only needing to produce 33% more frames for 40fps vs 30fps. Compare that to producing 60fps which is 100% more frames. So you are only sacrificing a small amount of performance to get 40fps but a significant amount to reach 60fps.
In practicality, most people can't tell the difference between 40fps and 60fps, or at least won't notice it in normal gameplay.
So you get nearly the same experience as 60fps but for much less processing power and therefore also better battery life, less fan noise/cooling, and you can crank graphics a little higher.
Most people are more likely to notice the difference between "Medium" and "High" very easily or "High" and "Very High", but won't notice the impact of 40fps vs 60fps. So that's why 40fps is generally a sweet spot between performance and experience.
In practicality, most people can't tell the difference between 40fps and 60fps, or at least won't notice it in normal gameplay.
So you get nearly the same experience as 60fps but for much less processing power and therefore also better battery life, less fan noise/cooling, and you can crank graphics a little higher.
Most people are more likely to notice the difference between "Medium" and "High" very easily or "High" and "Very High", but won't notice the impact of 40fps vs 60fps. So that's why 40fps is generally a sweet spot between performance and experience.