Red algae are red because of the phycobilin pigment phycoerythrin.
This pigment reflects red light and absorbs blue light. Blue light penetrates water to a greater depth than light of longer wavelengths
These allow red algae to photosynthesize at somewhat greater depths than most other algae.
Coralline red algae are important in the formation of tropical reefs
In some reefs red algae have contributed more to reef structure than other organisms, even corals.
Reef-building algae secrete a hard shell of carbonate around themselves, in much the same way that corals do.