Why is equalization necessary during descent?

Equalization is necessary during descent primarily to avoid ear and sinus discomfort. As a diver descends, the pressure around them increases, which can create a pressure differential between the environment and the air spaces in the ears and sinuses. If equalization is not performed, this pressure imbalance can lead to discomfort, pain, or even injury, such as barotrauma. By equalizing, divers equal the pressure in these air-filled spaces with the surrounding water pressure, allowing for a comfortable and safe descent.

The other choices, while related to diving, do not directly address the function of equalization during descent. Improving swimming efficiency is important for diver performance, but it is not the purpose of equalization. Preventing equipment failure is crucial for safe diving but is not a reason for equalization itself. Increasing underwater visibility is beneficial for a successful dive but does not connect to the need for equalizing pressure in the ears and sinuses. Thus, avoiding ear and sinus discomfort is the primary and most accurate reason for the necessity of equalization during descent.