Just to be clear, I'm only referring to activities that are victimless, or at least harm nobody except for those who willingly participate. If I understand Calvinist (TULIP) theology correctly, criminalizing something perceived to be a sin won't save anyone because refraining from that sin isn't enough to avoid hell. On the other hand, for someone who has already been born again, it is impossible to lose their salvation no matter what sin they commit. I don't mean to start a debate; I just want to gain some perspective.
In part, I think it is the desire for all to follow God's law. There is a sense that things will go better if we follow God's law.
I am not sure if this argument is brought up enough, maybe because it is challenging to specifically identify this, but a victimless crime is hard to fully define. At one point or another, the actions of the individual will begin to have an effect on the community at large. Pornography is often considered victimless since it is mainly the adult who engages with it. However, more and more research is being done on the negative effects of watching pornography, like rewiring your brain into a more juvenile state. Some of the main effects of porn is more depression, worse mental health, and lower quality of life. Or the effects it has on marital quality, which is negative. At some point a society addicted to pornography will have victims, either the people partaking or the way those people begin to act in society at large. This is not even to mention the potential for abuse within the porn industry, which recently stoked a huge response.
Now, this can be a slippery slope. Its easy to claim an individual action has societal effects, but actually putting legislation in place for something like this is extremely challenging. And yeah, you are right, just having laws in place won't give anyone their salvation, but it could help create environments conducive to considering faith. Eliminating pornography would be a huge help for many who struggle with it or those who will potentially struggle with it. Similar arguments could be made about certain drug uses. I think it is valuable to have a strong emphasis on the individual in these cases, but also to consider how it may impact a community.
The question at the core is, "how will this law provide environments for human flourishing?" And Christians often respond with God's laws as being conducive to human flourishing.
The logic behind the campaigns for legislation to enforce God's commands (or at least God's commands as understood by the people doing the campaigning) is the idea that a country does better and thrives, and will be blessed by God, if it follows God's law. The theological basis for this is found in various Old Testament passages where the nation of Israel is blessed when it follows God's Law, and punished when it does not. (I simplify considerably here).
The theological position is complicated by the fact that Israel made a specific covenant with God to follow these laws, and that they come from the Old Covenant, not the New Covenant of Jesus. Also the main laws Isreal is punished for breaking specifically concern worship of God, which few campaigners are pushing to make compulsory. In any case, no modern nation is in this situation, not even modern Israel.