I teach math - mostly to undergrads. As of late I have started using slides more and more. The reasons (and my rants):

 - Our sciences building is being renovated, so the math department is "temporarily" relocated in social sciences building. Thus we need to use whatever sorry excuses for lecture halls they, the nearby law school, and the adjacent educational sciences building have to offer. In a metrosexual climax those had largely opted to "modernize" and only equip their auditoriums with video projectors and stamp sized whiteboards, whereas I used to fill up a set of 8 blackboards of total area 6ft x 30ft every 45 minutes.
 - Students have been complaining about the legibility of my handwriting for quite some time. Moving to the whiteboards made the problem worse. Also, the standard of cursive taught in our schools has changed a number of times since I did time in grade school. Thus my handwriting is a twice removed cousin to what the students expect.
 - Try as I may, my 3D-sketches suck, which makes it a bit more difficult to explain my points in, say, multivariable calculus. When I leave rendering to Mathematica I have extra tools at my disposal. Such as full control of the perspective and color - colored chalk has always been hard to find (when the need springs up), and some colors are hard to see on a whiteboard. Also animations become feasible.
 - I have always been complaining about how the students concentrate on frantically taking notes instead of trying to follow my thought process. Thus it is simply intellectually honest to provide them with copyable documents about those examples that I really want them to spend time on. 
 - I have not observed noticeable dips in the attendance as a consequence of me using slides. Some freshmen are intoxicated about their newly achieved "academic freedom", and cut a class or three. But the choice of method of learning the material is their responsibility (and usually they learn quickly enough). My role is to provide them various ways of achieving that goal. 
 - Also, is there a significant difference to copying the notes from the course web page as opposed to from a "designated writer" class mate? If anything, the quality of the copy I provide is probably more useful. If somebody missed a class for family reasons or illness, they have a sporting chance of keeping up, when the lecture slides are available.