One reason is because dog-whistles can work: I have from time to time had the experience of expressing my opinion about a subject in a way that causes the minority who agree with me to recognize me as a potential ally without triggering reprisal from the majority who disagree with me.

Another reason is to preserve some credibility in case of a future discussion where I'm more willing to deal with the consequences of public opposition. Rather than having to say (for example) "Well, yes, I know I said policy X was a good idea, but I didn't really mean it; I was lying then, but you should totally believe me now because I'm totally telling the truth" I can instead say (for example) "I said that policy X is an efficient way of achieving goals Y and Z, which it absolutely is. But I don't endorse maximizing Y and Z at the cost of W, which policy X fails to address at all."

Yet another reason is to use plausible deniability as a way of equivocating, when I'm not sure whether to come out in opposition or not. That is, I can disagree while maintaining a safe path of retreat, such that if the degree of reprisal I get for disagreeing turns out to be more than I feel like suffering, I can claim to have been misunderstood and thereby (hopefully) avert further reprisals.