Some people have asked if this allows you to pull 30A from a 15A circuit. The short answer is "no." What confuses them is the conflation of units.

There are three units of interest for this discussion:

1. Volts. Volts measure the electrical potential between two points.
2. Amperes, or amps for short. Amps measure the current. Current is the amount of electricity that flows in a circuit.
3. Watts. Watts measure the flow of power. Power can be thought of simply as voltage times current (for loads with complex impedance it's not that simple, but that's beyond the scope of this discussion). Power is the rate of flow of energy.

So the question remains - how is it that two circuit breakers rated for 15A cannot be combined to make 30A? Why bother with this if you don't get more power?

And that question is its own answer. By combining the two circuits, you do get more power. Twice as much (assuming split phase rather than three phase), in fact. But that's because the voltage doubles, not the current.

To understand how 240V power works here in North America, think of two lumberjacks working a two-man saw. When one of the lumberjacks is pushing the saw forward, the other is pulling it backward. At the end of the stroke, the two reverse. Each man is contributing one lumberjack's worth of power. The net result is 2 LP (lumberjack-power - compare to horsepower, another unit of power). But crucially, the saw doesn't move any faster than it would if only one lumberjack was working it.