Primidone is a barbiturate analogue, and is metabolized to phenobarbital. Barbiturates are well known for the sedation they cause- that was and probably still is their primary use.

oops, scratch that, epilepsy has to be phenobarbital's primary use, and butalbital is primarily used to treat migraine.

The rest of the enormous class are primarily used for sedation and sleep.

A few are effectively relegated to lethal injection as they were so heavily abused and sought after that they just aren't used by physicians anymore. Benzos are just so much better.