They create a stable brine-in-oil emulsion.
They improve high-pressure, high-temperature (HPHT) fluid loss.
They provide a stable rheological.
They are used in conjunction with primary emulsifiers to create a stable invert emulsion
They also contribute to improving HPHT fluid loss, electrical stability, and providing a stable rheological.
Both primary and secondary emulsifiers are resistant to high temperatures, salt, and calcium invasion. They contribute to well wall stability, provide good lubricity, cause less damage to oil and gas formations, and ensure good mud cake quality.

The principle of action for both is the same, but they refer to different values of the index. They need to be matched with each other for use3. The primary emulsifier is usually used for creating a water-in-oil emulsion, while the secondary emulsifier is used for oil-in-water emulsion.