Modern CPUs already have dedicated crypto instructions which are pretty fast, without the cost of copying to another device's memory, loading a program, waiting for it to execute, then copying the output back.

https://en.wikipedia.org/wiki/AES_instruction_set

More likely audio/video workflows where encoding/decoding within a FPGA could be really useful. Realtime effects is also an interesting opportunity (e.g. similar function to the DSP accelerators on Universal Audio equipment).

1. https://www.uaudio.com/uad-accelerators.html