Phenobarbital is classified as a C-IV med, that's because it slow-acting and less likely for someone to get addicted too. Barbituates are usually categorized by the "swiftness" of their effects are felt by a user. They are categorized into four categories, Slow-acting (phenobarbital) class-IV, intermidate-acting (Barbital) class-III, short-acting "pentobarbital (nembutal) or secobarbital (Seconal)" these are C-IIs, and Ultra short-acting "Thiopenatol, aka truth-serum and is also used in minor dental procedures (intravenous anesthetology) also a C-II. It's the short-acting ones that are more dangerous, since they act so quickly and have strong pronounced effect, that places them at C-II level. And pretty much not prescribed nowadays, cuz you have "Restoril" and "Ambien", which are just as effective, but safer.
But in hospitals the short-acting ones can be used for those people who have been in freezing water (like a near-drowning type of situation) for a longtime like 20-30 minutes, and their core body temperature has dropped. Then the short-acting ones are given to keep those body temperatures low (since barbaituates lower body temperature), and allow a doctor to slowly raise the body temperature back to normal.
It's the classic diving response, all bodily processes slows to a crawl, this allows the body to survive an otherwise fatal outcome. So the short-acting and ultra short-acting one have their place in medicine, just not at your local pharmacy. Phenobarbital is still used to treat certain types of epilepsy and it is also a good sleeping-med and cheap.