I wish I activated this button shape long back. This is very cool. One of the benefits I see it increases visuals for buttons across the apps and also underline text on buttons.
To activate this option, go to: Settings -> General -> Accessibility -> turn ON button options.
One issue Apple should fix is that contrast for cancel button on compose window of mail app. It appears as blue text on black when button shapes are turned-on. Screenshot below!
Overall this is a cool feature for people with low vision, elderly and cognitive disabilities.