Why do even some of the big budget films slow down the frame rate to give slow motion effect rather than use a slow-motion camera? The point is that the first technique almost always give a hint of "ruggedness" which is totally smoothed on using a proper slow-motion camera instead.
Slow motion video is recorded in a rate higher than the playback rate (Ben already explained the details). When taking a still image (but it corresponds to video) there are 3 parameters:
Modifying these 3 will result in certain effects:
When modifying the frame rate you're modifying the exposure time. Respectively you will need to modify either or both the aperture or the sensitivity, which may result in effects you might not want (shallower depth of field or more noise).
Additionally and probably most important sometimes you can either not modify the latter two parameters or you may hit a certain limit where you can't modify them so you need to introduce more light sources or more powerful light sources which will usually effect the overall "feel" of the shot.
There's always more reasons like "I want that ruggedness" which is totally viable in my opinion because you may create interesting effects.
slow motion happens by filming at a higher frames per second (FPS) than the playback on a projector happens. What you're talking about is a high speed camera, designed to capture more FPS than a standard camera. If a high speed camera records at 240 FPS and play back runs at 24 FPS, then what took 1 second to record will talk 10 seconds to watch, and be very smooth. If you record 48 frames per second and playback at 5 frames per second it will still take about 10 seconds but won't be very smooth at all.
Some high speed cameras record at 12,500. They're expensive and really only good for this one job.