Why are two cameras mounted in paralle in stereo application? For stereo cameras on the market, two cameras are always mounted side by side and with a displacement that is perpendicular to the cameras’ optical axes. I take this setup for granted. One idea came to my mind whether this is necessary? If two cameras are not parallel and have different focal length, camera calibration can correct the difference. Why are two cameras mounted in parallel? My guess is that two cameras can have a large overlapping region. Am I correct?
When you say cameras are parallel  it only means that the center pixels are parallel.

Every pixel has a different angle, and that refers to the overlapping regions you were talking about. Actually there is at most one overlapping voxel (3d pixel kind of) for every 2 pixels!

There is more about it in epipolar geomtry , it's interesting and pretty simple!