What problem does this address?

The buttons block provides four alignment options which does not include all the potential values for justify-content in a flex container. I find that the space-around value in particular can be a really nice way of centering and filling available space, and so don't know why it's not available.

What is your proposed solution?

Add support for justify-content: space-around to the Buttons block. Consider also adding justify-content: space-evenly for nearly full coverage of all possible values.