Although many of them are not harmful, some of them do cause diseases by living parasitically in hosts and causing skin infections. However, it is also beneficial to man in that it decomposes dead plants and animals. The atoms from the decomposed organisms return to the ecosystems where they can be used once again.
We eat members of the Plantae kingdom. Eubacteria live in our digestive systems, decompose dead organisms, and are used for genetic engineering. Some Protista members are food for aquatic organisms and we eat organisms in those food chains. We are in the kingdom Animalia. We eat some other members.