What is the purpose of /usr/libexec?

Executables are stored in /usr/libexec on Unix-like systems. The FHS says (section 4.7. /usr/libexec : Binaries run by other programs (optional)": > /usr/libexec includes internal binaries that are not intended to be executed directly by users or shell scripts. Applications may use a single subdirectory under /usr/libexec. On macOS, rootless-init a program called by launchd immediately after booting, is stored in /usr/libexec. Why would it be stored in /usr/libexec when it is a standalone executable that could be stored in /usr/bin or /usr/sbin? init and other programs not called directly by shell scripts are also stored in folders like [/usr]/{bin,sbin}.