When you describe objects through a microscope why why use micrometeres(um) instead of milimeteres(mm)?

What units of measure you use depends on the scale of the object in view. You would choose a scale so the number is greater than 1 unit and less than 1000 units. The units are for a size reference. It does not make sense to choose centimeters when you are looking at the width of a human hair, for instance. Just as you would not use micrometers to measure your height. It is easy for you to ociate numbers like 1 or 10 or 100 but difficult to ociate numbers like 0.1, 0.01 or 0.001. I recently used a microscope with several different lenses. Each was inscribed with a reticule, which is a ruler, a calibrated scale so you can eyeball the size of the object in view in real terms with out an actual mechanical measuring device. The scale inscribed was set to accurately indicate the size in that particular lens magnification.