Notice that Paul calls the deeds of evil and darkness as "worthless." There is really no lasting satisfaction in sin! Moses is said "...to share the oppression of God's people instead of enjoying the fleeting pleasures of sin..." Hebrews 11:25 NLT. Sin does bring "pleasure" to the flesh, but it's backlash is horrible. Both Moses and Paul knew that immorality could only lead to a wasted life and they wanted no part of it!
The idea of "exposing" these deeds as worthless is to encourage believers to avoid this kind of behavior. They are meant for greater and more noble things and their time and efforts would better be spent living for the Lord and gaining eternal rewards, 1 Corinthians 3:14 NLT, "If the work survives, that builder will receive a reward." The devil wishes to ruin many a Christian by encouraging them to engage in behavior that not only robs them of blessings, but brings on them trouble, despair, and hardship. Note the contrast in Jeremiah 4:18 with Jeremiah 5:25. In 4:16, 17 Jeremiah tells the people of Judah they will suffer defeat from their enemy and then in verse 18 he stays, "Your own actions have brought this upon you..." Over in Jeremiah 5 we read in verse 24 of God's blessing of good weather for good crops, but in verse 25 Jeremiah tells the Judeans, "Your wickedness has deprived you of these wonderful blessings. Your sin has robbed you of all these good things." What believers, young and old, need to remember is that sin destroys so much and keeps believers from enjoying God's blessings that would come from obedience to God.
Paul then goes on to say that "exposing" sin is not to go into detail about the sin itself. It is shameful to speak of sin in a graphic way. Far too many preachers have violated this principle in their careless and detailed description of sensual sin. As an old saying goes, "You don't need to remove the lid off the septic tank to know that it stinks inside!"