What) Why do some trucks have an extra set of wheels that don't touch the ground?
It sounds
like you are talking about a drop axle. You see them on dump
trucks, logging trucks, cement trucks, and on some cargo
haulers. They can be lifted when the truck is running empty
and does not need the extra weight capacity the drop axle
affords them. This also cuts down on friction and adds fuel
mileage because the wheels only need to be carried and not
propelled. When the axle is in the down position, 2 small air
bags that lift the axle are vented and 2 large air bags are
filled with air from the trucks accessory tank. The large bags
will allow the axle to support 2000 lbs of weight. There is a
regulator valve that allows the driver to adjust how much air
pressure is delivered to the bags and how much weight the axle
supports. These axles also have brakes and passive steering
capabilities. The extra brakes allow a heavier load to slow or
stop easier and the passive steering capability is controlled
by 2 large hydraulically dampened coil 
springs mounted to
and parallel with the axle and tied into the steering cross
tube arms. Hope this helped. (1)
The main reason these wheels are retractable is that the vehicle cannot make tight manouvers with these wheels extended. Due to their positioning they slide sideways instead of rolling.
A fair number of long-distance motor coaches (eg: Greyhound and Trailways) have these drop axles as well. One nifty thing that the bus builders are doing is designing the suspension so the drop is automatically lifted when the transmission is in reverse or if the forward speed is under 15 MPH to improve maneuverability.
Due to
state laws you will see LOTS of these in Michigan. Don't know
the details, but it was something that really stood out when I
had to travel there on business. (2)
Where) In or near Ontario
Why) Pam
and I had fun speculating on the purpose of these extra wheels
while we were driving and the explanation above certainly
makes sense to me. The mystery that remains is why did
we seem to see twice as many in southern Ontario or in the New
York/Michigan area when we were close to the Ontario
border. Are we just not very observant or is there
something else going on? 
It is hard to take one of these pictures when Pam is speeding (probably) down the highway. When you are passing the truck you are too close to take a good picture (see above) but when the truck is going the other way it is hard to tell if it has the extra wheels until it is nearly too late. I took that kind of picture of this truck in Pennsylvania.
This truck was stopped at a gas station in Buffalo, New York so I was able to take a couple of better pictures.
