In short: Anodes supply (almost only) metal ions - brightness comes from organic additives. These additives are not formed at the anode and are continuously consumed or degraded in operation. That’s why the bath needs regular brightener replenisher.

Why anode replenishment isn’t enough

• Anodes dissolve metal (e.g., Ni²⁺, Cu²⁺), keeping the metal concentration constant. Organic additives (carrier/suppressor, brightener/accelerator, leveler) come from outside - not from the anode.
• Consumption at the cathode: Additives adsorb to the surface; some are co-deposited or electrochemically reduced/decomposed. This depends on current density and Ah throughput.
• Degradation at the anode: A portion of the organic components is oxidized there (especially in chloride-containing baths or at high anode polarization).
• Side losses: Drag-out on parts/racks, adsorption in the filter/anode bag, thermal/chemical breakdown and purification (e.g., activated-carbon treatment) remove additives from the bath.

Role of the brightener replenisher

• It typically contains the short-lived, highly active components (often the “accelerator/brightener” fraction) that are consumed fastest.
• Without make-up additions, the deposit loses brightness, leveling and fine grain; dull areas, higher stress or roughness may occur.

Conclusion

The anode replenishes the metal, the brightener replenisher replenishes the functional organic additives - both are needed for consistent, bright deposits.

Note on bath life

While, in theory, regenerable electrolytes could run indefinitely via anode dissolution, other additives are consumed. To keep using the expensive electrolyte, those additives are topped up. Still, without special purification, the electrolyte will not last indefinitely - with proper additive maintenance, its service life can be increased many times over.