I will assume that you want to get the answer to why cameras have an objective-eyepiece combo instead of a simple lens, that is with the combined power of both.
I will start with a single lens system, where you have a lens with fixed focal length.
Note here that the light sensitive film needs to be placed at a point where light rays converge together. So, when you want to take an image at infinity, you would keep the film at focus of lens. In other cases at other places.
That effectively means the distance need to be changed between the lens and the CCD, the light sensitive part in a digital camera. This can pose an engineering problem, since this adjustment of focus can be very delicate and a small deviation can hamper the quality of photo considerably.
Now let's move on to a 2 lens system. Here, what you have is a moveable objective lens and a stationary eye piece towards the CCD sensors. So now the previous problem is eliminated.
So, now one has to only change the distance between objective and eye piece to get images at any distance. This is the movement what you see when you actually zoom the camera, lenses come out and try to zoom onto the image.
In modern cameras the lens system consists to half a dozen or so lenses. As @Nic pointed out a double Gauss system contains 7 lenses. This is to further reduce the imaging defects like chromatic aberration and coma formation.