To paint a better picture of how this is so useful, imagine using a microcontroller to both interface to a camera and perform image processing in order to make a decision (maybe you are trying to track an object). In terms of cameras you are able to interface to, you are left with those with slower serial interfaces (SPI, UART). If you really wanted to interface a microcontroller to a camera with a faster parallel interface, you would have to spend a little more money to get a fast enough controller. But this doesn't leave your controller much time to do anything else other than image-processing and frame-grabbing! On the flip side, if you were to use an FPGA it becomes harder to actually implement your decision making and takes longer to develop and is harder to debug.
Having an FPGA and Microcontroller on the same IC solves this exact problem. Now, you can develop your frame-grabbing for your higher-speed camera and even do some image processing on the FPGA. In this way, the FPGA can be treated as a custom co-processor for your application running on the Processor. And the FPGA to Processor interface is seen by your application is either 'just another memory mapped peripheral' or even an 'external interrupt'.
The example I used comes directly from my project, where I did use the FPGA to communicate with a faster camera (well faster than UART or SPI based) and allow the processor to dictate what kind of operations to perform on the image, or even stream the image to the processor's RAM for streaming to a remote PC via UART. This was for an Autonomous Robot project, which I will add posts on in the near future.
I recommend checking this technology out. It is great to see innovations such as this, because they have the power to take industries into new directions. If you are interested in getting yourself one, I suggest either the Zedboard or the Zybo.
Link to Digilent's Zybo