Never understood the point myself, unless you've got a garden and you think it makes a meaningful difference there. Most of my compostable waste (food waste) goes down my garbage disposal. Not sure why composting is better.
Using a garbage disposal and dumping your waste down the sink is much worse for the environment. It means that your compostables break down anaerobically, which produces an awful lot of methane and then eventually a thick sludge. This sludge gets screened out of the waste water treatment plant and dumped in a landfill. Not really a good scene. That doesn't cover the water waste that you have to use to flush the stuff down the drain.
Composting in your back yard:
- Is aerobic, so doesn't produce the quantity of greenhouse gases that using a garbage disposal does
- Doesn't end up in a landfill
- Doesn't waste water
It's complicated, like most things. Composting is probably better in most cases, but it's not always convenient or something people want to do at home. Municipal compost collection takes care of this, but not a lot of places offer that.
Putting things through a garbage disposal can be good or bad, depending on your plumbing system, the local sewage system, and what they do with their waste. Some places go as far as to collect it and generate energy, some places are probably just dumping it all back out relatively untreated.
As people alluded to earlier, stuff doesn't really "compost anyways" in a garbage dump the way people think. Trash facilities have to be designed not to allow uncontrolled decomposition for safety and stability reasons, and things get weird with lack of air, etc.
I've composed at home, using a large bin, small containers, a worm system, and considered an electric composting thingy; as you can tell it was more of a hobby and out of curiosity. They all worked well, produced compost at a reasonable rate and got rid of waste, but I'm not sure if I'd go through the hassle of setting different systems up again somewhere new, and would hope the municipality would take care of it.
I think the most efficient system would be for me to be feeding my foodwaste to the dogs, and then composting their small amount of waste (yes, I know pet waste has to be composted at higher temperatures). It would keep the dogs happy and save me money on dog food :)