Why is the secret ballot?

Does voting need to be secret?

When I’ve asked someone who they voted for, he answered “I’m not telling you” when I asked why he said “because it’s a secret ballot”
Update: I made a error, I was meant to ask “What is the secret ballot?”
Update 2: Sometimes, I just ask because I was curious. I am not personally someone who would judge who voted for what and didn’t care whenever or not he was pro EU or not.
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