Loose ends can be tied up or left alone. The process of unraveling starts with a single thread working itself loose over time and can end with an entire piece of fabric becoming un-done. They are best addressed quickly.
People have loose ends, too. Maybe a relationship that has been neglected or a project that was started that was never finished. It's important to address those because they serve as mental anchors that can slow you down over time as the uncompleted thoughts or projects begin to pile up. Over a lifetime, it amounts to an enormous amount of unfulfilled potential and a dragging sense of what could have been.
This is why it's important to reach closure. Finish what you start or move on smartly.