The flash ROM chip is mounted on the mainboard either by being soldered to the board, or by being placed in a socket.
Socketed chips can be removed/replaced with just a little physical force, while soldered-on chips need to be desoldered somehow.
(Soldering is the process of connecting two metal objects using a third metal that is melted in the joint and then hardened. Compare welding which means the two metals themselves melt and then harden.)
When developing BIOS software the build/run cycle includes reprogramming the flash ROM that the system boots from. During development the BIOS will not allow the system to complete the boot process and so the system will not be fully usable until the BIOS is debugged. This means that the flash ROM must be reprogrammed in another system, or that two flash ROMs can be switched between so that the system can be started both with a known good BIOS and the developing LinuxBIOS.
Being able to remove the flash ROM easily from the target system simplifies development greatly. There's also a product called the BIOS savior that enables simple switching between two flash ROMs, it is designed to be mounted in a socket.
Bottom line: flash ROM in socket is a requirement for BIOS development, otherwise you have only one try to get everything right, which is unlikely to impossible.
//Peter