Safety glasses should be worn any time you are doing an experiment, especially one that involves chemicals or chemical reactions. They prevent chemicals or other materials from getting on or in your eye, and can prevent anything from mild discomfort to permanent blindness.
Some pairs of safety glasses have magnifying glasses on them, similar to bifocals. They can be used to more carefully examine something in an experiment.
Meat contamination because humans do not eat air or fertilizer so meat contamination is the only logical answer.
