I know that Intel acquired Altera in 2015 and the new Intel chips include a built-in FPGA. But what is it for?
But what is it for?
It's a bit early to say for sure. As of May 2018, Intel has only demonstrated one FPGA/CPU part (the Xeon Scalable Processor 6138P), and it's not yet shipping to the general public.
That all being said, the theoretical advantage of having an FPGA in the CPU package is high-speed, low-latency data transfers between the CPU and FPGA fabric (and, potentially, to connected devices). One of Intel's example applications is software-defined network switching, for instance.
"Anything" you want for which you cant afford the MIPS otherwise. There is a lot of effort in "non traditional" industries, such as Brokerages, that are using FPGAs/ASICs to execute trades faster, as latency equates to dollars.
Also, Intel is pushing the OpenCL (higher-level "Configurable Logic" language) for use in their (formerly Altera) tool flow, to lower the entry point for "consumer-side FPGA use". This is not going to change the ASIC/FPGA commercial/professional world, but will allow consumers to create custom logic. Like when you want to try and cheat on your non-server-side FPS game maybe(?) ;)