Here's why:

* Gravity is the only force acting on the projectile vertically. Once launched, the projectile is only affected by gravity in the vertical direction. Air resistance is often ignored in basic projectile motion problems.

* Gravity acts downwards. This means the acceleration due to gravity is always directed downwards, regardless of the projectile's initial velocity or trajectory.

Key Points:

* Value of g: On Earth, the acceleration due to gravity is approximately 9.8 m/s².

* Direction: The vertical acceleration is always downwards, even if the projectile is moving upwards.

* Constant: The acceleration due to gravity remains constant throughout the projectile's flight.

Important Note: This assumes we are dealing with idealized projectile motion, where air resistance is negligible. In real-world scenarios, air resistance can influence the vertical acceleration, but it's often a minor factor compared to gravity.