Back in 2013, Intel 8920 did cost $750 wholesale, and similar Broadcom card a little bit cheaper.

Looks a lot now, but back then you didn't have server CPUs with gazillion cores on the cheap.

It was also about getting more bandwidth to a single server. 20Gb/s was a practical limit per mainstream server, but with accelerators you could've multiplied it many times more than the fastest CPU on the market.