Because the PA view or the X-ray taken when your standing facing the X-ray machine may either not show an abnormality or an abnormality may need clarification on the lateral view. The lateral view is taken when your side is facing the X-rays. The most complete or best x ray for diagnosis is the one with 2 views. Usually a PA and Lateral view.
For example, shadows on X-rays are produced by anything impeding the path of the X-rays. Tissues along the chest wall such as the breasts can impede the X-rays and produce shadows. This is especially tricky when the nipple casts a shadow that looks like a nodule. A nodule can represent a cancer. A lateral view will show that there is nothing in the lung, but it is the chest wall or breast that’s casting the shadow.
Another example is a pneumonia that can hide and be subtle on the PA view in the right middle lobe of the lung. On the lateral view, you can usually see a clear pneumonia overlying the heart where the right middle lobe projects. At times, lung nodules can only be seen on the lateral view. I have seen mediastinal masses only seen on the lateral view of the chest.
Abnormalities of the vertebra are also best seen on the lateral view. While the thoracic vertebra are seen on the PA view, they are seen in profile on the lateral view. Abnormalities like compression fractures or vertebral lesions are best seen on the lateral view. The sternum is also seen in profile on the lateral view.
Pleural effusions or fluid in the lung can also best be seen on the lateral view, especially when there is a small amount of fluid. Fluid trapped in one of the lung fissures can also best be seen on the lateral view sometimes. Abnormalities of the hila or where the vessels enter and leave the lungs can sometimes also be best seen on the lateral view.
It also goes that if something is only seen on the lateral view, the PA view can clear it up. Sometimes normal structures can cast confusing shadows on the lateral view, especially near the heart and chest wall. In cases of doubt, a chest CT can be done which will be definitive.
Therefore, the best initial test of the chest is a 2 view chest X-ray consisting of a PA and Lateral view. This will provide the most information in a quick and relatively inexpensive way. A 2 view chest will also prevent unnecessary further testing in many cases such as CT. There is less radiation exposure then a CT.