The reality is that the sound of bell helps to create a unity in the Left and Right parts of our brains and thus increase the concentration especially for devotional purposes. The moment the bell rings, it produces a sharp sound that echo for minimum seven seconds. The duration of echo is good enough to activate all the seven healing centers in our body. This results in keeping the brain from all negativities.
In nutshell temples have bells for peacefulness, calmness and sharpness of the mind.