The most often form of tattoo today is esthetic (in a broader sense of the term). People tattoo themselves to express their views or sense of beauty or make a memento of something. But there is also a practical variant of a tattoo: a medical tattoo. They can have different purposes and we'll mention some of them here.
Some medical tattoos are used on patients that suffer from a chronic diseases or allergy that, when they strike, can render patient unable to speak and tell what his or her problem is. These patients have name of their ailment on some part of their body.
Tattoos are used to mark parts of the body as an aid in radiotherapy. These marks help to adjusting the beam accurately which lessens the radiating of surrounding tissue and damaging.
Some army troops tattoo their information (like blood type) on their bodies. U.S. military does that on the rib cage bellow the armpit (and in the military fashion these markings are called "meat tags"). Waffen-SS in Nazi Germany of the World War II also did something like this. They tattooed their blood type underside of the left arm, usually near the armpit.
When an areola has been removed during mastectomy or a breast reduction surgery, an areola can be tattooed to help patient recover better.
In ancient times people (we have mummies from the Neolithic times, ancient Egypt, China and other places that have tattoos) also used tattoos as a primitive variant of medicine and as a form of acupuncture