I'm just curious: but why exactly do I have to transfer my BTC to another wallet?

I understand that this is a standard phrase for safety reasons in the FOSS world.

But shouldn't BTC and BCC co-exist nicely? So will this procedure do my BTC any harm or the sky will fall on my head or something?

I just want to be sure that the BTC wallet will be okay like in a "nothing did touch me" way and the other wallet will just use this extracted private key to get the BCC...