Why is it important to keep a microscope covered when you're not using

This prevents the microscope from becoming covered in contaminants, such as dust particles in the air and even microscopic things. While they may be invisible to the naked eye, they are large under a microscope. Covering it also keeps it in a better shape for longer and can even provide protection if (and let's hope this never happens) one were to be knocked off of a table.

Particles and other things can get in it or if someone messes with it

Dthrough a slow process