Mantras are the Buddhas in the form of sound. And it is easier to recite than to visualize the Buddhas Even looking at the Buddha, we may not be able to discern all the qualities of the Buddhas. Hence, mantra in comparison is holistic, easy and hence powerful.
When reciting mantras we can recall the qualities of the Buddhas, in particular the qualities of Buddha that we are reciting the mantra of, generate in our mind these qualities; usually it will involve Boddhicitta/compassion, wisdom, increase etc. If we then abide it these qualities in our mind it will affect our entire day. It is this that will protect our minds.