You probably meant that technically, even the trees like ginkgo or all the oaks and maples imported from afar are useful - they can withstand the ufriendliness of city life. I agree. It is just a different kind of usefulness that potato and cucumber have, and back then there was no way to weigh costs and benefits of introduction. (The usefulness of a cactus is yet another thing.) Or you could mean that having a rich botanical garden was a status thing for a capital; an obligatory research facility for a self-respecting university. It should be still somewhat true. However, today people know little enough about native species that there's merit in educating them, and the libraries now get it backward. Or you could mean, further, that botanical gardens used to be efficient institutions of progress, and indeed gave rise to centralized experimental biotechnology research as scientific approach. That is, I think, probably true.