In short, FPGAs are good where you need to perform a little processing on a lot of data, and CPUs are good where you need to perform a lot of processing on a little data.
An HDMI video stream is a lot of data. It can be done by a CPU, GPU, or ASIC in the general video case, but if you need to do a little bit of work on it (add an overlay, for instance) you might choose an FPGA.
An audio stream isn't a lot of data, but if you need to perform speech recognition on it, you're going to prefer a CPU to an FPGA.
While you can do software defined radio in a CPU, you can deal with a much larger portion of the spectrum with an FPGA more easily than in a CPU.
While you could make a keyboard controller out of an FPGA, a microcontroller is going to be cheaper, consume less power, and easier to develop advanced keyboard software (macros, gaming functions, remapping) for than an FPGA.
Of course you can do anything in any of them, with tradeoffs, but if you are proficient in both then you will be able to weigh the tradeoffs more competently, and will avoid the higher cost of parts or development time you'd incur choosing the wrong solution to a given problem.