Why are phycobillins are classified as accessory pigments?

Phycobillins : (a) They are the another group of photosynthetic pigments found in red algae and cyanobacteria.
(b)They are protein linked pigments hence, are called phycobilli proteins. Red algae contain a red coloured phycobillin called phycoerythrin, while in cyanobacteria it is blue coloured, phycocyanin.
©Like chlorophylls, phycobillins are tetrapyrrole but do not contain magnesium and phytol chain.
(d)Phycobillins are found inside sub-microscopic structures called phycobillisomes attached to thylakoids.
(e)The light energy harvested by these pigments is not directly utilized but is transferred to chlorophyll ‘a’ in the manner similar to carotenoids.
(f)Therefore, phycobillins are also classified as accessory pigments.