From WWF website:

The MPA is located adjacent to the Agulhas Bank, representing an important area that contributes greatly to the high biotic diversity of this region. The intertidal zone within the MPA has faunal elements representing both warm-water east coast species and cold-water west coast species. The resulting richness and diversity of intertidal organisms attracts a large variety of fish species to this coast.

De Hoop MPA has been very successful in protecting populations of sought-after reef fishes, and in providing migrant recruits of over-fished fish species such as Red Steenbras to neighouring fishing areas. 

The De Hoop MPA is currently the only conservation area that affords protection to the unique intertidal system of large, eroding, soft sandstone and limestone platforms.
Three species of fish known to be present in the MPA are listed as “vulnerable” .These species are the great white shark (Carcharodon carcharius), white steenbras (Lithognathus lithognathus) and red steenbras (Petrus rupestris). 

De Hoop MPA is critically important for the conservation of the Southern Right whale (Eubalaena australis) which is an endangered species. De Hoop MPA, together with St. Sebastian Bay contain 70-80% of cow-calf pairs on the South African coast and ranks as probably the most important nursery area for Southern Right whales in the world.

From Cape Nature website:

The De Hoop Nature Reserve is a Marine Protected Area (MPA) on the southern Cape coast. Visitors come to the dunes of Koppie Alleen from all over the world for a glimpse of some of the reserve’s special guests – the whales.

The highest level of protection is applied at the De Hoop MPA to limit human impact to a minimum. Not even a rock or little shell may be removed from here, but if you’re willing to walk a bit, you can also enjoy this area.

The De Hoop MPA does not only protect the reserve and the coastline. It also stretches three nautical miles into the sea, meaning that all these little sea creatures are also protected.