This is a rear exhaust fan adapter to accommodate an aftermarket 80mm fan (Noctua specifically though others will likely work). Without such an adapter you can still install a fan but there is a large gap which reduces the airflow through the HDD bays. This piece mostly seals the gap improving airflow dramatically.

Designed for: StarTech.com HSB4SATSASBA 4 Bay Aluminum Trayless Hot Swap Mobile Rack Backplane for 3.5in SAS II/SATA III - 6 Gbps HDD