Just because you can't think of a reason doesn't mean they don't exist.
Here's a list of people you forgot:
People who regularly trailer cars or heavy equipment for whatever reason
People with non-European sized campers - see below
People with large boats - see below
When it comes to boats and campers, you are not taking wind resistance in to consideration. Many campers in the US are much taller than the tow vehicle causing massive amounts of wind resistance. This is a consideration along with weight. Same with any large boat. If it's sticking way above the roof of the towing vehicle, it probably shouldn't be towed by your sub-compact or little crossover.
Any trailer with brakes should be pulled by a full-size truck or SUV, especially if it's towed on a regular basis. Your little car may say it has a certain towing capacity, but that is assuming optimal conditions and new parts. If you are towing on a regular basis, you *will* put more strain on the drive train, suspension, and brakes than someone who doesn't. Most cars, minivans, and hatchbacks these days seem to be built as cheaply as possible so the extra wear and tear does make a difference. Vehicles that are purpose-built for this kind of work are generally heavy-duty enough to handle it.
It makes little to no financial sense, and is wasteful, to own a dedicated towing vehicle if towing is something you do even a handful to times a year. Most families can't afford more than 2 vehicles, so if towing is something needed and both adults are working, the tow vehicle needs to double as a commuter.
The only heavy haulage work involves moving concrete, sand, or building supplies, and if that's your gig then you need a light commercial vehicle.
At least in the US, full-size trucks are the light commercial vehicle of choice and are often driven to job sites by their owners. You can't really expect a construction worker--think of an independent contractor who for sure isn't making tons of money--to own a separate work vehicle.