Note to avoid confusion: this refers specifically to certain Canadian airports with preclearance facilities. In other cases, e.g. flying out of Québec City, you have to clear stateside.
The US pre-clearance at Canadian airports also allows much more flexibility for the airlines: they can then fly into any US commercial airport as a domestic flight rather than having to go to an ‘international’ airport with customs facilities. Coming back from the States, however, you end up among the herds using the regular Canadian customs facilities.
Wow - thank you for the information. I’ve never seen that, nor have I seen any pre-clear lines marked. I wonder if they unload in a different arrivals hall? 
I need to take eleven different prescription drugs each day. When traveling, I keep them in unlabeled baggies (Each day’s worth is together, four days per baggie, seprated by twist ties.) I keep enough for the trip in my carryon, and a second set in my checked luggage. I’ve crossed the Atlantic many times and have never had a problem.
You might not have had a problem yet, but all it takes is one grumpy security person to ask you what these drugs in baggies are, and you can’t prove their identities or the fact that you have a prescription for them. IMHO, you’re asking for trouble!
Ask your pharmacist for smaller containers with a valid label on them if the original containers are too large… then you can transfer what you need to valid temporary containers, and still have proof of drug identity and validity in having them with you.
In Toronto, you go through the US customs/immigration/whatever preclearance, and are then in a separate area of the airport that only has passengers going to the States. Planes that pull up at the gates there can go anywhere in the States and land as a US domestic flight. So at the destination, you end up passing through domestic arrivals.
The Wiki article mentioned that immigration-precleared flights from Ireland still have to go through customs and agriculture inspections, so they still land at international terminals. Which would seem to defeat the point of the whole thing.
Actually, that seems to be pretty standard among all countries, visa waiver or not. That’s a good one to point out.
That’s my take on it, and if so I still don’t see the point. I typically get through passport control very quickly and am waiting for up to half an hour or more just to get my stupid bags. Then you have to go through customs, and re-check the bags, then go back through security…
On my recent trip to Taiwan, on the return I had to pass through 6 fucking security screenings. One at the entrance to Taipei airport, one at the gate itself, then landing in Osaka we had to all get out, go through security, so we could sit…about 100 feet from the plane. Then get up and go back through security again, then land in the US, be screened coming in, exit the secure area to go to an unconnected terminal, then go back through. At least I only travel First Class on the international flights, but it’s still just a stupid fucking pain.