This was added in 5.5 to accommodate addons that had traditionally depended upon Global Scrapbooks to function.

You can see this github pull request
https://github.com/concrete5/concrete5/pull/395...

There was a forum thread about it somewhere.

Basically it's so that blocks can be accessed in addons by name using

Does this therefore allow for selective display of blocks within a given Stack?

So, for example, a stack could be created which contains several navigation-related blocks, but to display them separately in the theme Block::getByName() could be used?

It always feels a little wasteful to me to create an entirely new stack when I know I'll only be putting one block into it (probably silly, I know, but it doesn't feel very efficient!)