Microscopes allow humans to see cells that are too tiny to see with the naked eye. Therefore, once they were invented, a whole new microscopic world emerged for people to discover. On a microscopic level, new life forms were discovered and the germ theory of disease was born. The organization of an organism's body from cells, to tissues, to organs, to systems, to the complete individual could finally be studied and visualized.
The Cell Theory emerged from early work with microscopes. The notion that all living things are composed of cells, that cells are the units of structure and function of living things and that cells arise from pre-existing cells were all powerful ideas.
Microscopes allowed scientists to observe Prokaryotic cells which make up Bacteria and Archaea. These cells are small and contain no membrane- bound organelles. It allowed them to observe Eukaryotic cells with a nucleus and membrane-bound organelles that perform different life functions. These are seen in Fungi, Protists, Animals and Plants.
Ever since the first simple microscope was invented, as technology improved and led to the development of compound microscopes, electron microscopes and beyond, our knowledge of the microscopic world has expanded tremendously. Society has reaped the benefits new knowledge can provide in the fields of medicine, surgery, pharmaceutical industry, biology, taxonomy, etc.