Who can tell how many lives have been saved because of medical alert bracelets. But it can be safely said that many owe their lives to the medical staff who were able to quickly find out their medical history because it was written on their bracelet.
If you need special medical care because you have a condition such as diabetes or epilepsy or other condition that needs to be taken into account in an emergency then you should have this on your alert bracelet. This is especially true if you are not conscious or otherwise aware at the time of your medical emergency.
If you are allergic to medication such as penicillin you want your medical staff to know this in case you are not able to communicate with them. The medical professionals feel more confident treating people with medical bracelets when that person is unable to communicate with them.