said by Nick_L:why WARM bleach?
Probably just because it would have a slightly greater "washdown" effect, i.e. the bleach kills the algae/mildew and the warmer it is the better job it does of carrying away the dirt in the line(s).
My bleach bottles are in an outside Rubbermaid storage unit, so my bleach is already VERY warm (it's 100deg here all summer)!