Linked from
The 7 pages that link to Holdfast, each with the reason it gives.
Kelp forestBroader topic: Kelp holdfasts attach the forest’s foundation to rocky seafloors.
KelpBroader topic: Kelp use holdfasts to grip rocky seabeds against waves and currents.
SeaweedBroader topic: Many seaweeds use holdfasts to stay attached to rocks or other substrates.
PorphyraRelated: Porphyra blades attach to rocks or other substrates by a small basal holdfast.