Why should you watch from the side before you lower the objective of your microscope?

Sometimes, if the stage is set high enough, the objective lens can hit or scratch the slide when it's changed, because each objective lens is a different length. From the back of the microscope, you cannot see if the lens is going to hit the slide or not, and run the risk of disturbing your sample. This problem can be prevented by looking from the side and stopping if it looks like the lens is going to hit the slide.

Watching from the side also helps to know if it is too low, or just in the right distance.