you need to go to the newbie room with questions like that. They are so basic its clear you didn't bother to look for tutorials before asking.
You never flash the original bios unless they are damaged.
The original bios do not allow backed up games, storing games on the hard drive, replacing the hard drive or blocking macrovision. The modchip bios enable all that.
The evolution X software helps you utilize what the bios allow.
(Yes, you need to flash your modchips bios before using an ISO of Evolution x)