A topical anesthetic is used during applanation tonometry to numb the surface of the eye, specifically the cornea. This is important because applanation tonometry involves gently flattening a small area of the cornea to measure intraocular pressure (IOP), which is a key test for diagnosing and managing glaucoma. The procedure can cause discomfort or a reflexive blinking response if the eye is not numbed. By applying a topical anesthetic, the patient experiences minimal discomfort, allowing for a more accurate and efficient measurement of IOP.