To speak against sin is important. Then those who hear and who are not Christians but want to be free will know that freedom is in Jesus and come running to his open arms. And they will receive help by receiving salvation. They come realising that otherwise, they are helpless.
But many preachers do not want to preach against sin because they themselves are enmeshed in it. They are afraid that if they preach too much against sin they might be exposed as a fraud. These people become the gates through which different sins find entrance and place in the church.
Some are not enmeshed in sin but they do not preach against it because they do not want to offend people. They do not know that what they permit, they enforce by default. They have become partakers of other people’s sin.
By not speaking against it they give the people permission to continue in the error of their ways. They will be judged as if they commit the sin. The judgment is not only for those who commit the sin but also for those who give others permission either covertly or overtly to continue. If you will not speak out then the blood is on your head, you will bear similar blame, even if by yourself you are as clean as can be.