People look for the drip inside the house and seal the spot directly above it. Water travels. The crack might be 4 feet up the slope. If you only seal the point directly over the stain, you missed the hole. You must seal the origin , not the symptom .
The plugin is available for roughly $39 per year as a standalone extension or as part of a larger architecture package. Instant Roof Nui Crack
You can often find a trial version via the SketchUp Extension Warehouse or the developer's site to see if the tool fits your workflow before purchasing. People look for the drip inside the house