Nothing in real life stays clean or perfect- not for long, anyway! "Decals" are a great way to add visual detail, breakup tiling textures, and tell a story in your scene, with very little performance cost.
Examples include:
Drips, dirt, and grunge
Blood splats
Wear and tear or decay
Bullet holes, burn marks, etc
Paint, graffiti, posters, etc
Signs and markings
QuickDecals 2 makes it simple (and fun, honestly!), to add these details. I recommend watching the quick video to get all the details, or read through the official documentation, but here's the basic idea:
1) Drag-n-drop textures to QuickDecals
2) Set Per-Decal settings
3) Pack Atlases to reduce drawcalls
4) Select one or multiple Decals (for randomizing) and "Shift + Right-Click" to place