I'm a cleaning and stain removal master and my arsenal does include magic erasers.  

Whether your picking out paint, countertops, tile, stain for wood floors, etc those colors always seem appealing when a buyer is walking through.  

It looks great when the building is brand new, but without a good power wash every year or so, the aesthetics deteriorate quickly - dust will accumulate on horizontal surfaces, then get washed down the face by rain - leaving ugly dark stains down the side.

Certain paints resist staining and scratching.  

Stain it?  

And I always walk through property and take pictures of everything at the start of a lease term, especially if there are known existing damages like current stain in carpet or small crack in tile, then provide tenant list and tell them they can add issues to it within x days (2 weeks) if they find other problems moving in, and then I either repair/replace or put it on the list so they aren't responsible for it at lease end.           

Hard to stain, and if there are stains, they can be lightly sanded to remove the stain (plumbers got PVC primer on one tub during construction that has to be sanded out).  

The hardwood floors were stained from dog urine and the whole house smelled like dog piss.  

A ceiling stain caused by a leak?  

While out "driving for dollars" my partner and I noticed alot of properties with black stains / streaks on the roof.