Unexcited synchronization is used where multiple generators are required to black start and energise a load comprising transformers, where the rating of the transformers is many times the rating of the multiple generators.
It allows comparatively large transformers to be energised by multiple generators, without subjecting the generators to the usual inrush current of the transformers.
(Reference: Case Study - Unloaded Transformers Energised by a Generator at 11 kV; Effect of Magnetising Inrush)
It allows a smaller generation capacity to be installed, instead of a capacity which could withstand the inrush of the transformers at rated voltage. Unexcited synchronization also reduces the mechanical stress on the transformers through the ramped excitation of the transformer.
Unexcited synchronization may be the either the primary or secondary means of synchronization for the generators.