Death certificates are most helpful when researching your family history. Valuable information can be learned from a death certificate. Information varies from state to state and from year to year. Death records prior to the mid 1800's are rare. By the early 1900's most states kept fairly good records.
Death records can be very helpful in uncovering a genetic disease.
Based on the cause of death, you can determine whether medical testing maybe necessary. Each state has their own laws regarding the release of death certificates. Some states like New York require a person be deceased for at least 50 years before releasing a death and you must be a direct descendent or immediate family. However, alternate sources do exist that still make obtaining a record legally possible. Death record archives vary from state to state.