An attendant should NEVER leave their post without a qualified replacement.
OSHA first instituted regulations regarding Permit-Required Confined Spaces (1910.146) because of the high number of serious injuries and deaths in confined spaces. Entering these spaces is very dangerous business, and the attendant serves as the “safety watchman” for entrants as well as those who may try to enter. This also applies in an emergency situation when others may be tempted (but unqualified) to enter the space to rescue a co-worker.
For the complete list of the Duties of Attendants according to OSHA, view the regulation page here.