Why does anyone ever need an FPGA? 

Ross Freeman and some Zilog engineers came up with the idea that a microcontroller should be software definable then started Xilinx in 1984 - I think they still own like 50% market share of the FPGA market https://i.imgur.com/a6CX64F.jpg

creating and destroying; built flying cars & autonomous vehicles, now building roc.camera, writings: july.rocks

There are many different reasons, but to generalize: a CPU/MCU is not performant enough but lack of update-ability or cost rule an ASIC out.