Why do Magnoliales seeds usually contain abundant endosperm?

In the order Magnoliales, seeds typically contain abundant endosperm because it serves as a vital source of nourishment for the developing embryo.1

In these primitive angiosperms, the embryo is often minute and relatively undifferentiated at the time the seed is shed. Because the embryo is so small, it requires significant additional development before it can rupture the seed coat and produce a seedling. The copious endosperm provides the necessary nutrients to support this extended period of post-shedding growth. This contrasts with more advanced angiosperms, which often feature larger, more developed embryos and reduced or absent endosperm.1