Carbon Monoxide Detectors - why's it so vital?

We have 4 carbon monoxide detectors in the house.  When we installed a new woodstove, the city required we have detectors in the room with the stove.  So we did.  The next week they upgraded their requirements so that we have a detector (at eye height or higher) next to the bedrooms.  So now, we have all these Carbon Monoxide detectors and apparently they expire after so many years and won't quit bleeping.  Time to get new ones.  

This made me realise, I don't understand why it's so important to have these annoying bleeping things that want their batteries changed twice a year.  We already have smoke alarms.  I'll have them because I'm supposed to.  But I'm curious: what are the actual dangers?  Why are these so important?  What makes them different than smoke detectors?  Why are these so much louder than the regular fire alarm?


What I know of Carbon Monoxide
- it's a heavy gas
- it makes us sleep a sleep we never wake up from
- it comes from cars
- and somehow it comes from fire
- it's a bad thing for humans

My 10 seconds on google came across this interesting link.

CO is a by-product of incomplete combustion of fuel such as natural gas, propane, heating oil, kerosene, coal, charcoal, gasoline, wood, or other bio-fuels. This incomplete combustion can occur in any device that depends on burning a fuel for energy or heat.

My fire is inside a cast iron box.  Are there things I can do to minimize CO gass entering my home?

If CO is a heavy gas, why does the city want us to install the detectors up high next to the smoke alarms?  

CO is odourless and tasteless, the first symptom is usually a headache which people don't suspect of being CO poisoning- they'll go to sleep it off but never wake up.

CO isn't heavy, its about the same density of air (slightly lighter I think)- but it is produced as a byproduct of combustion, in hot air- and hot air rises.

Even a fire inside a cast iron box isn't air tight, the fire gets air from somewhere. By comparison a boiler is usually sealed-vent, it has a vent to the outside from where it gets its air for combustion, and from where combustion ages are exhausted, it has to be really faulty for CO to enter the house. Very few wood stoves are air-sealed and get all their combustion air from outdoors (though you can get external air kits for some models, they don't tend to be airtight either)

As an aside my CO detectors last 10 years with a sealed battery, I certainly don't have to change them every 6 months! The brand is Kidde.