It Works as a Cheap Backup Mirror Cover
Proper mirror covers exist for a reason. They fit better, hold tighter, and are less likely to blow away than a loose plastic bag. If you regularly park outside in winter, near trees, or in dusty places, real covers are the better long-term choice. They are made for the job.
But travel is rarely perfect. Maybe you forgot your cover. Maybe you rented a car. Maybe you are staying overnight somewhere cold and only have what is in the trunk. In that moment, a simple bag can become a useful backup. Tie it carefully, make sure it is not scratching the mirror housing, and remove it before you leave.
That is the real value of the trick: it is quick, cheap, and easy to remember. It gives you a temporary solution when you need one. Solo travelers often learn to appreciate small hacks like that because there is no one else to run back to the room, scrape the mirror, or hold a flashlight. A bag is not elegant, but on the right day, it can be useful and surprisingly practical. Pack one anyway, but do not rely on it blindly either.