This is part of a chain of connections that allows a host computer to communicate with the enclosure to access its power, cooling, and other non-data characteristics.
This overall approach is called SCSI attached enclosure services:
The host computer communicates with the disks in the enclosure via a Serial SCSI interface (which may be either FC-AL or SAS).
The disk-drive then communicates with the SES processor in the enclosure via ESI.
The above sequence is just a simple implementation of a 4-bit wide parallel interface which is used to execute a SCSI transaction.