According to the video linked somewhere in this thread indicates WhatsApp Erlang workers that want sub-ms latency.

It's definitely for ads auctions

It's Meta. They always push to be that fast on paper, even when it's costly to do and doesn't really need it.

Meta is a humongous company. Any kind of latency has to have a business impact.

If you have 50,000 servers for your service, and you can reduce that by 1 percent, you save 50 servers. Multiply that by maybe $8k per server and you have saved $400k,you just paid for your self for a year. With meta the numbers are probably a bit bigger.