Respirator fit testing is recommended for use of all RPE facepieces including disposable devices.
The AS/NZS 1715 Standard states an individual shall be fit tested prior to initial use, whenever there is a change in your facial characteristics or other features which may affect the facial seal of the respirator and should be repeated annually thereafter. 
Under the various pieces of health & safety legislation ensuring a respirator fits correctly is considered a reasonably practicable step to managing an individual workers safety.

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Humans come in many shapes and sizes, as do respirators. This results in wide variability of physical dimensions and features of both people and respirators. The ability of a respirator to form a satisfactory seal or barrier between the wearer and the contaminated environment may be significantly affected by these variables. 
If the respirator-user match (fit) is not checked, an unsatisfactory seal/barrier may unknowingly exist. This could allow excessive leakage of airborne contaminants into the wearer’s breathing zone, even though the user is wearing a respirator correctly selected for the application.
A fit test is used to assess whether a specific type, model and size of respirator can adequately fit a specific individual.