Flashing roms

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http://forum.xda-developers.com – Apologies if this has been posted elsewhere, I can't seem to find an answer to the question I want to ask. I've just upgraded to my HOX and have flashed it with the latest venom Rom. Everything is ok, but what I'm confused about is why do we need to go into the downloader everytime we want to flash a new Rom. What I mean is why does the boot img file have to be extracted from the custom Rom and then loaded via a command prompt? I'm not complaining by the way. Would just like to know why this is the case. On my desire hd it was just a case of rebooting into recovery and then flashing. (HowTos)