Difference between a Dual camera phone and a single camera phone
Dual Camera Setup on smartphones provides results which are hard to achieve through single camera setup for e.g. DSLR like bokeh effect, better white balance and low light performance, 3D image, optical zoom with no moving parts etc.
Bokeh is the word for Blurred background, the one you see in photographs taken by professionals through expensive DSLRs. Bokeh is affected by a few factors: the lens design, the relative depth of field, and the aperture of the lens. Dual camera setup can provide information like distance measurements, and depth of field data which can be in turn used by a software to give Fake bokeh effect to your photos.
Better White Balance and Low Light Performance
Smartphones cameras starts struggling when they have to perform in low lighting conditions, one solution for this problem is using the larger image sensor which is hard to incorporate in a smartphones where every single millimeter is very precious in their design. A larger sensor may result in a bigger camera bump on the smartphone. In case of Huawei P9 or Xiaomi Mi5s Plus’s dual camera lenses one lens has RGB sensor and other one has monochrome sensor. They may look the same, but they have very different functions. One shoots in RGB to produce a color image, while the other takes care of the monochrome (black and white) images. The monochrome lens brings out the detail, and lets in around 200 percent more light than the RGB lens which takes care of the colors. The result is photos with very good white balance, contrast and lesser noise.
Two cameras can capture two different photos from different angle to make a 3D image.
optical Zoom with no moving parts
iPhone 7 Plus’ camera is a good example of it, you are able to zoom on the iPhone 7 Plus without sacrificing image quality. The dual-lens, provide two focal lengths, with 1x and 2x magnification. There's no moving parts ( hence it reduces thickness of the smartphone) required for zooming in or out by changing the focal focal length of the lens, it simply switches between lenses of different focal length.