I don't understand why anyone would buy it

Optical quality, build quality, and overall durability. The EF 17-40mm f/4L USM is an "L series" lens -- essentially professional grade, while the EF-S 18-55mm f/3.5-5.6 is a consumer grade "kit" lens. L lenses are made with better materials, better designs, and more features. They're weather sealed to keep out moisture and dust. You'll notice that the maximum aperture for the 17-40mm is f/4 across the entire zoom range, whereas it varies between f/3.5 and f/5.6 on the 18-55mm. Most importantly, L lenses tend to have better optics -- sharper focus and fewer optical flaws such as chromatic aberration. Also, the "USM" designation on the 17-40mm stands for "ultrasonic motor", i.e. the autofocus motor is nearly silent yet very fast.

One other important difference is that the 18-55mm is an EF-S lens. The "-S" means "small image circle" -- lenses designated "EF-S" instead of "EF" produce a smaller image at the back end, inside the camera. For that reason, and also because EF-S lenses may project a bit further into the camera body, EF-S lenses only work on crop-sensor bodies (the Digital Rebel series, the x0D cameras, and the 7D. The smaller sensor doesn't require as large an image as larger sensors, and the smaller image in turn allows lenses to use smaller components. That means that EF-S lenses can be smaller, lighter, and less expensive than their EF counterparts. Note that the 18-55mm also includes image stabilization, while the 17-40mm doesn't.

To answer your title question, the difference between focal lengths of 17mm and 18mm is small enough that you could consider the two lenses to be about the same, focal-length-wise, when used on the same body. (On the other hand, 17mm on a full frame sensor is a lot wider than the same lens on a crop sensor. 17mm on a crop sensor gives the same effect as a 27mm lens on a full frame sensor.)

By the way, you might also want to compare the EF-S 17-55mm f/2.8 IS USM to your kit lens. The focal range on that lens is almost exactly the same as the 18-55mm, neither lens is designated "L", both offer image stabilization, and yet the 17-55mm costs about the same (a little more, actually) than the 17-40mm. How can that be? It's pretty much the same answer -- despite some similar specs, they're different lenses. The 17-55mm has a constant f/2.8 aperture, so it needs larger lens elements that collect more light. More glass means a heavier, more expensive lens even though it's an EF-S lens. If you want a similar lens at f/2.8 on a full frame sensor you need even more light and bigger lens elements, so the closest equivalents would be one of the two EF 24-70mm f/2.8L lenses, at $1800 or $2300, respectively.