This is particularly true in the land of commercials, where if you're doing a 15-second spot, and if four seconds of that gets used up with live action and shots of the logo, then you've got a total of about 11 seconds of animation. For an example, watch the commercial above, which Bent Image Lab made last year. In those 11 seconds, the commercial packs in 9 shots of animation, many of which are about half a second long.
So if no one's really going to see these things, why take the time to make them look good? Well, for one thing, somebody made the shelves, and somebody else made the back wall. A half-hearted job on the shelf items would tarnish the good work that the other artists have done. So part of the motivation behind doing good detail work is respect for the detail work everyone else is doing.
But what about the shelf items that we never see onscreen? Why bother making them at all? Oftentimes, the storyboard won't reflect the exact shot the director decides to use, and so first of all, it's unclear exactly what will be seen until they start shooting, so it's best to make things as if they will show up. Also, it's a matter of giving the director options. If a background element looks like crap, the artist who made it has essentially made a decision for the director that that element will not show up onscreen.
And ironically, in the land of commercials where most props get maybe three seconds of screen time, it is especially important for the art department's work to be impeccable. The client has 15 seconds to convince the TV audience that they NEED this product. There's no cushion time for the spectator's mind to wander. The mind control has to be perfect and complete. So if someone's watching a commercial and thinking, "Wow, that brick wall looks like they printed out a pattern from Google images..." then the commercial fails and it's the art department's fault.
See, I bet you never thought that art department peons like me are the titans that hold on our shoulders the global consumer economy. Well, now you know. So don't honk at me when I ride my bike on Division Street.