The importance of the defensive aspect
Knowing how to attack is important. But being able to defend well is at least as important. To perform at an expected level in both these areas, a wing-back must be in top physical condition and have outstanding cardio capabilities. Otherwise, it will prove impossible to sprint up the field and then back as required by the position.
Wing-backs must also be able to position themselves correctly relative to the centre full-back, providing support on the sides. During defence stages, when the opposing team has the ball, they must provide coverage to protect their goal and prevent the opposing #9 or winger from finding open space ahead. And when their team has the ball, they must also make themselves available and help move the ball up the field. Whether it's staying close to the centre full-back or #6, supporting the winger, attacking the space from deep, etc., wing-backs have many options open to them.
In short, the evolution of the wing-back position over time means that to perform well at that position you must be a well rounded player and have both defensive and attacking abilities. It's a big challenge!