In a theatre (or any illusion-creating event), if the sound and visual image don’t match, the audience’s brains spend unnecessary time trying working out what is going on. As a result (and without necessarily realising it) their level of engagement in the performances drops.
Even more annoyingly for a sound designer, the sense-perception of our eyes overrides that of our ears, even when we know our ears are correct. The McGurk Effect illustrates this clearly.
Sound has a great responsibility to maintain the illusion so that what you see and what you hear match.