FPGAs are often used to test/design not a "circuit" but an ASIC (application-specific-integrated-circuit) that you will then go on to actually build. Or, if your application doesn't have volume to suppprt the ASIC engineering costs but can support the FPGA unit cost, you just leave it as an FPGA. There are millions of devices (industrial machines, research, testing, etc) where this makes sense.
And not all FPGAs loose state when powered down. For those that do, there's typically an easy way to hook up a little Flash chip and bootloader to reload the state automatically on reset. For those that don't have that option (EG, currently working with an ABB robot, Mecco laser, and Rockwell PLC which all contain FPGAs, only the Mecco can boot itself), you store state in battery-backed SRAM and just leave the RAM on at all times.