If you're traveling somewhere with extremely restrictive drug laws (e.g. Singapore), you can be sentenced to prison for drug particles on your shoes. i.e. you can accidentally have forbidden substances on you.
It's not that they " can sort of smell drugs", they are extremely sensitive to any trace smell of drug. Your linked article confirms that:
Dog-handling officers and trainers argue the canine teams' accuracy shouldn't be measured in the number of alerts that turn up drugs. They said the scent of drugs or paraphernalia can linger in a car after drugs are used or sold, and the dogs' noses are so sensitive they can pick up residue from drugs that can no longer be found in a car.
Search so thoroughly that you would even find the tiniest crumbs left on someone's shoe sole and that 44% "success" rate would probaby jump up considerably.
Though the false positives that are mentioned in the article are also an issue, but probably less so if you're regularly doing the rounds at airport waiting queues.