The microscope is important because biology mainly deals with the study of cells (and their contents), genes, and all organisms.
Some organisms are so small that they can only be seen by using magnifications of #xx2000 - xx25000#, which can only be achieved by a microscope. Cells are too small to be seen with the naked eye.
Genetics is the study of variations in an organism generation after generation. Genetic engineering requires mixing of genes. Genes are even smaller than cells, which is why microscopes are essential to genetics.
Without the microscope, biology would not have been so developed and many diseases would still have no cure.