My hope is to persuade others that the initial idea is false and/or harmful, not to prevent others from hearing the bad idea.
If I succeed, the bad idea still exists, but others can understand why the idea is bad and recognize similarly bad ideas in the future.
If you simply censor the bad idea, at least two bad things happen as a consequence: 1) the bad idea is not refuted, and so is likely to be reintroduced later with fewer people who understand why it is a bad idea; and 2) the person who originated the bad idea does not have any reason to change their mind, but rather is more likely to resent being censored and so hold that bad idea more tenaciously and attempt to spread it more surreptitiously.
I am frankly frightened by this relatively recent promotion of the idea that preventing the public utterance of bad ideas and distasteful speech is good for society. Based on my understanding of history, it is, in fact, a precursor to totalitarian society.