Why is custom recovery a prerequisite for flashing a custom ROM? I have read in many guides that before flashing, we need to have a custom recovery like CWM installed. I am not able to understand why is this a prerequisite for flashing a custom ROM. Can I flash a ROM without custom recovery?
Stock recoveries usually check the update.zip file being flashed for a specific signature that is only known by device's manufacturer and/or carrier.  You cannot therefore flash custom ROMs on stock recoveries, hence the need for a custom recovery which bypasses said check.