Installing vanilla LFS has all its perks and upsides, alongside an educational benefit. This book aims to solve another problem, where users may need to build or run something for 32-bit on a 64-bit machine, where 64-bit is the architecture they built LFS for. Section v, “About the Multilib Edition” covers why this may be the case and how multilib works.
MLFS is recommended to be installed after having installed LFS at least once beforehand to be better equipped with the knowledge of how LFS is supposed to work.