The spleen is an oblong organ just below the stomach. A pet can live a completely normal life without a spleen, however the spleen is a storage area for blood. If a pet had a severe hemorrhage, for example, the involuntary muscles of the spleen contract, squirting forth a fresh supply of blood. The spleen also removes old red blood cells from circulation.
There are three major reasons to remove the spleen. The first is a mass on the spleen. Masses can be benign or malignant. It is not always clear prior to surgery. Sometimes the veterinarian may check to see if cancer has spread to the lungs with an x-ray to determine if it is malignant, but lack of evidence does not mean its benign.
A second reason for removal is bloat. When the stomach twists in bloat it can cut of circulation, but it also can twist involving the spleen. Frequently the spleen is damaged and must be removed in part or fully.
The third reason is traumatic rupture. If the patient has had blunt force trauma to the abdomen such as being hit by a car or kicked by livestock the spleen may rupture. It can bleed dangerously. If a tear in the spleen is small it may be repaired. However, if it is severe it needs to be removed.