So what are they for? If you scrape your plates so there are only tiny bits of soft food going down the drain you don't need a garbage disposal anyway. I don't have one and I've never had a blockage.

A clog doesn't quality as something going terribly wrong. Empirically, my plumbing costs are $300 over 10 years. By comparison, rats are actually a big problem in the deep wooded suburbs. We spend a lot more on exterminators, rat traps, poison, and mesh covers to keep them out of the house. Rats smell bad, scare the girls, attack the chickens, spread garbage around the pickup area, spread fleas to the dogs, and are a nontrivial vector for serious human diseases.

So given that rats are a bigger problem than clogs under my current policy of scraping plates down the disposer, I should probably do it even more.