Imagine a situation where your mistake, your failure, even your sin, somehow results in something good for God's kingdom. Does that mean the sin itself is okay?
Paul is addressing a tricky objection here. He's just argued that God's faithfulness is magnified, not diminished, even when people are unfaithful (Romans 3:3-4). This leads someone to argue: If my 'lie' (my unfaithfulness, my sin) actually makes God's 'truth' shine even brighter to His glory, then why should I still be judged as a sinner?
This isn't Paul excusing sin. Instead, he's exploring the logical extreme of a flawed argument. The objector is trying to deflect responsibility by saying, 'If my sin serves God's glory, then I shouldn't be blamed!' It's a bit like saying, 'If my mistake leads to a greater discovery, then the mistake wasn't really a mistake.'