DSLR Manual Film Mode? D3100

Why do people set their DSLR to Manual mode and then set the camera's shutter speed and aperture?
Doesn't the camera already have a shutter speed when you set it to 30 or 60 fps? And the aperture is set by the camera automatically like f/6 or f/11?
All I thought one could set on a filming camera was the exposure (and maybe aperture)…
And can I set the shutter speed and aperture on my Nikon D3100, when in Manual mode?

With respect to video, the digital camera has the same options as it would for shooting still photos. The shutter and the frame rate are independent variables. The frame rate merely determines the timing for each exposure to be taken in sequence.

The high speed shutter helps reduce light levels in very bright scenes, and capture motion with less blur. When trying to get depth-of-field selective focus, you need to maximize the aperature, so using the high speed shutter does this. The camera does not take another frame immediately after the shutter cycles, but rather waits 1/30th second before re-arming the shutter. It is a kind of intervalometer action.

Unlike motion picture film, digital cameras can reduce the effective shutter speed to less than the frame rate. CCD means charged coupled device. Normally, light builds a charge on the CCD pixel element, then the charge is amplified, "read" by an A/D converter and discharged. However leaving some of the charge on the CCD after each frame is read allows it to build to a higher level in the following frame. Sony night shot averages 4 frames, super night shot averages 8 frames. However these slow shutters have the same effect as for still photography, it is difficult to prevent blurring.using a tripod is mandatory.

the other adjustment you have ignored is setting the ISO. Using a low ISO number improves image capture because it reduces the elecrical amplification needed and electrical amplification adds noise. In traditional video cameras this is the "gain" setting, on dSLR cameras it is called ISO, both refer to exactly the same thing.