It is probably more relevant for battery packs than single cells. NIMH batteries will self discharge over time. Some cells in a pack will self discharge faster than others meaning that some cells will be completely dead and some at say 50% capacity. That means, that if you first charge them using a fast charger, some cells could be over charged before the charger shuts off due to peak detection of the deadest cells. Or the opposite could occur with some cells only being partially charged due to peak detection of the 50% charged cells.. A slow 16hr (0.1C) equalization charge allows safe over charge of the partially dead cells while allowing the other fully dead ones to charge 100%.