> Aren't FPGA's used mostly to test/design a circuit that you would then go on to actually fabricate/build?

We use FPGAs in cell sorting because we need to make decisions off of high dimensional data with low latency. The cells moving through our system have velocities higher than 1 m/s. They flow past a set of lasers and wind up in a droplet less than a millisecond later so we need to make a decision whether or not to sort a droplet within that time frame. We don't use ASICs because we don't move enough volume to justify the startup cost.