In which case is it preferred to use init or systemd for managing system startup?

init is a traditional initialization system in Unix-like systems that sequentially runs initialization scripts during boot. systemd is a modern init system that parallelizes service startup, manages dependencies, and provides advanced monitoring and management features.

Using init is preferable in simple or legacy systems where complex service management is not required. systemd is suitable for modern Linux distributions where fast boot, flexible service management, logging, and dynamic system state management are important.

Therefore, systemd is chosen for modern systems with high demands on service management, while init is used for compatibility or simplicity.