The way I understand it is that you can do a lot of DSP related work in the FPGA such that a Microcontroller board can do more sophisticated stuff than otherwise.

For makers and hobbyists, the power efficiency is less of an issue, in general. But FPGAs may enable a battery powered application where it wouldn't be possible otherwise.

Also, for learning advanced FPGA programming, audio and DSP seem to be a fruitful playground.