First off, what are microbes?
Microbes (also known as "microorganisms") are any living organisms that are so small, you need a microscope to see them. Some consist of only a single cell (like bacteria, archaea, and some fungi) and others are multicellular (like many protists and fungi). Viruses are also sometimes lumped with microbes even though most scientists don't consider them to be "alive." So what's the big deal with microbes? Well for starters...
Microbes are everywhere!
On our planet, there are an estimated 4-6 x 10^30 bacteria cells: that's 5,000,000,000,000,000,000,000,000,000,000 cells! From the depths of the ocean, to inside volcanoes, to the driest parts of Antarctica, to the deepest mines we've ever dug, we have yet to find a place on this planet that doesn't have life! Knowing how organisms live in these very different environments helps us understand a lot about the basics of how life works and can help us gain an appreciation of the diversity of life on our planet.
Microbes are old!
While animals have been around for a little over 500 million years, microorganisms covered this planet for at least 4,000 million years! In fact, the chloroplasts inside of plants (the organelles that enable them to do photosynthesis) were originally photosynthetic bacteria called cyanobacteria, in a process that eventually created animals, plants, and other multicellular life. Since all of us are descendants of these little critters, studying microbes (both extinct microbes in fossils and those still around today) helps us understand our own past and learn how we fit into the broader context of life on Earth and in the universe.
Microbes are important!
Did you know that land plants aren't the only things doing photosynthesis on this planet? In fact, about half of all the CO2 taken out of the air by photosynthesis is done by photosynthetic algae and cyanobacteria in the oceans. Microbes also help plants grow by fertilizing soil with nitrogen, help us clean up after oil spills, and are even used in an enormous number of the foods and medicines we make.