This group of single celled organisms with no organelles or a membrane-bound nucleus, constitutes the largest domain. Mostly different genera and species are distinguished on the basis of their metabolic talents. The importance of this group of microorganisms is hard to overestimate. Some cause disease, some provide services (like Lactobacillus and yogurt) and the capacity to decompose discarded organic products. The molecular machines in some bacteria are particularly spectacular. For example, the bacterial flagellum is famous as an example of irreducible complexity. Also, from bacteria we obtain restriction enzymes (important in pursuits like DNA fingerprinting and biotechnology) and CRISPR (important in almost every aspect of biotechnology including medicine) these days.
The amazing sophistication of the metabolic capacities of these tiny organisms is clear evidence that these organisms were created fully functional and important for the entire ecosystem.
Bacteria: Superheroes of the Microbial World (YouTube, 18 min) Secrets of the Cell, Episode 6 with Michael Behe