What is the difference between SoC FPGA and 'regular' FPGA?
As you said it has an arm core, they also tend to have a smaller amount of fpga resources compared to a similar price fpga only part.

It's neat that it allows the a direct access to the fpga fabric and if remember right it can also share a DDR interface between arm and fpga section.

It's nice if you need to run part of your application in software like maybe a high level portion of a network processing stack or the control plane for data processing while doing complementary things in hardware.  

If you don't need it though you're better off with a dedicated fpga.