Hi Andy,
I'm familiar with large scale heaters. The pressure relief is mandated for some pressure vessel design international codes for example for the case of the valves being close either side (in and out) and the liquid thermally expanding, for example due to electric heat, external fire, or sun etc. So apart from personal or plant preference, it may be mandated to have a PSV depending what code or standard the heater body was designed to.
Electric power can be interlocked to be off or not turn on based on a few variables, temperature within the heater being too hot (this protection is often built into a standard heater), zero flow, high outlet fluid temperature etc.
Dazzler