A few centuries ago, faithful Catholics considered "idle hands" (i.e. nothing to "do") a danger because one could be led into all kinds of distractions; and distractions were known (back then) to lead to sinful thoughts and ideas. On the rare occasion that they fell into idleness (they worked a lot more than we do), they would strive to keep themselves busy with either their hands or their minds. They did this to keep themselves from the influence of the devil (who loves an idle person). Back then, faithful Catholics worked to control their thoughts and keep them under the influence of holiness.
Today, we have a very different situation. Most people in the modern era, when they fall into an idle state, they look for distraction. They want music to fill the void (sometimes with no care that the words of the song are encouraging immoral behavior), or endless scrolling (knowing that they might see something that will promote immoral thoughts), or some other form of imaginary activity.
We treat boredom as an evil, and forget that if our own thoughts are mature and protected by godly hearts, that we can be content with our thoughts and not need someone else to pump stuff into our heads. It is amazing how quickly we allow strangers to tell us things and influence us. When the Scriptures tell us to "guard our minds" that means that we protect who can "wander in" and who we reject. Idle minds are dangerous; always have been and always will be. Let us guard our minds, and not seek distractions; especially those that can lead us off the path of holiness.