[css-align] A couple questions about CSS box alignment

I'm new to the CSS Box Alignment spec and I'm currently reading it from the
point of view of someone trying to understand it enough to imagine a
browser devtools for aligning elements on a page.

To that end, I have a few questions. I'm sure they're quite beginner level,
and I hope they're fine to ask here.

1) Why does justify-content not work on block containers?
It seems to me like that would be useful to align content along the inline
axis nicely.

2) I don't understand how align-content is meant to work with multicol
containers.
The alignment subject is the entire column box which, I assumed, would take
the full block size.
But I'm guessing that's where I'm wrong. Are column boxes different sizes?
Is this due to the break-* properties?

Thanks a lot for taking the time to help me understand!
Patrick