The steam purge is to prevent the resid from entering the valve cavity. If there is a steam pressure loss, leakage can occur, and it will enter the cavity (bonnet).
As with switch & overhead vapor valves, you need steam pressure to prevent the cavity from getting full of resid.
At an extra heavy crude upgrader I worked a few years ago, they do not have any leakage through their bottom delta valve (installed 2005). They did have leakage, through the bolted transition spool. A welded transition spool is recommended.