I can't see any useful reason to go to the absolute start of the line in 
the interactive shell. However, it does make sense in the source editor, 
and it is consistent with, for example, Visual Studio. The first use-
case off the top of my head is when you want to copy a block of code and 
not lose the relative indentation of the first line. You press home-
twice to go to the absolute start of line and select from there. It's 
also the only quick way in IDLE to be sure that the text widget is 
scrolled all the way to the left, since there's no horizontal scrollbar. 
(Any idea why that is? I assumed it's to discourage long lines, but I 
don't know.)

The proposal I refer to in IDLE-dev is this:
    http://mail.python.org/pipermail/idle-dev/2009-January/002742.html

I only mention it because it demonstrates another reason why we might 
want to override the default selection/navigation mechanics.