The diaphragm is partially closed down during darkfield microscopy in order to reduce the numerical aperture of the objective below the lower of the two numerical apertures inscribed on a darkfield condenser. This action preserves the darkness of the background in darkfield observation. The iris diaphragm is absolutely necessity for high numerical aperture oil immersion objectives (above N.A. 1.2) when using an oil immersion darkfield condenser. For ordinary brightfield observation, this iris diaphragm is left wide open.
In order to preserve darkness of the background for darkfield microscopy, the objective cannot have an N.A. higher than the lowest N.A. marked on the darkfield condenser. An iris that can reduce an objective's N.A., can allow you to use higher N.A. objectives for darkfield work. Objectives with an N.A. above 1.2 require an iris for darkfield. For ordinary brightfield observation, the iris can simply stay wide open.

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