It holds in odor and sometimes it hides stains from previous tenants. 

One of the top reasons landlords have trouble renting their properties is (brace yourself) - cat odors/dander.  

The floor has been stripped down to sub-floor and the sub-floor must still be holding odor along with walls, ceilings, and everything else in the house.

The odor does carry into other apartments and into the common areas.

I got a text from the tenant tonight around 8:30pm (it's Friday night) saying there's still an odor and that they scrubbed the carpet even after I shampooed it and they're asking if I can bring my carpet shampooer tomorrow.What do I do?

In general more pickups seems better to reduce odor and pests, but larger size may be more economical.

This prohibition includes, but is not limited to, loud noises, loud music, noxious or unpleasant odors, and disruptive behavior or actions."

Bugs and musty odors are a few potential small problems.

But I had one house where the odors were so bad (pets, not smokers) that I eventually had to have a drywall company re-texture all the drywall, then primer and paint.....

@Daniel Alfandre when the odor is really bad you need to replace the sub floor and the bottom foot of drywall along with all the trim.