Why hide your plate?
A visible license plate can identify a vehicle's owner and lead to personal information. Before posting a car-for-sale ad, hiding the plate avoids cold-calls, identity issues and scams.
Blur or black bar?
Gaussian blur and pixelation make the plate unreadable while staying visually subtle. The black bar is the most radical, clearest option for a listing. Both prevent the number from being read.
For garages too
If you post repair photos or customer vehicles, masking plates is good privacy practice toward your clients.