Door edge guards and window rain deflectors rarely make anyone's must-have list, but both prevent a specific, common kind of damage that's more annoying to fix later than to prevent now.
What door edge guards actually prevent
Door dings from your own door hitting a curb, a wall, or another car's door in a tight parking space — the edge of a door is usually the first point of contact, and it's also one of the more visible, hard-to-touch-up spots on a car. A guard absorbs that contact instead of the paint underneath it.
Installation reality
Most adhesive-backed guards need the door edge cleaned with rubbing alcohol first — skipping this step is the most common reason they peel off within weeks. Press firmly along the full length after applying, and give the adhesive the manufacturer's stated cure time (often 24 hours) before closing the door on it repeatedly.
What window rain deflectors actually do
Redirect airflow (and rain) up and over a partially cracked window, so you can leave a window cracked in light rain, or during a hot parked car, without water coming in directly. They also cut down on wind noise at highway speed with a window cracked.
The fit question
These are one of the more model-specific accessories — a deflector cut for the wrong window profile will either sit with a visible gap (defeating the purpose) or not seat into the window channel securely. Check that the product lists your specific make, model, and year rather than a generic "fits most sedans" claim.
Clean and dry the mounting surface thoroughly before installing either one — dust or residue under the adhesive is the single most common reason either product fails early.