Fact - Though the idol within a temple doesn’t require a bell at all, but they are there for a particular reason. The purpose of the bell is to help keep the chit of all the worshipers focused, so that one doesn’t hear the hustle and bustle outside.
Details - Similarly, other rituals like dressing and putting ornaments on the idols, lighting candles, incense sticks are all helpful in focusing the chit and meditating on the Lord. How thoughtful! Isn’t it?