What problem does this address?

In the editor, blocks with alignment (e.g. Wide, Full) are wrapped in a div with the .wp-block class and a data-align attribute.

All other block specific classes are then added to a child of this block.

<div class="wp-block" data-align="full">
    <div style="background-color: rgb(185, 204, 70);" class="wp-block-group has-brand-green-background-color has-background" ...>...</div>
</div>

This means that selectors such as .wp-block-group.alignfull.has-background have no effect in the editor.

This leads to two problems (that I know of):

1. Frontend theme styles often break and have to be recreated with specific editor stylesheets.
2. Sometimes, useful selector rules, particularly nth-of-type, nth-child, and adjacent operators, cannot be used in the editor.

For example, if you want to remove margins between two full-width groups with background colours you might do something like this:

.wp-block-group.alignfull.has-background + .wp-block-group.alignfull.has-background {
    margin-top: -50px;
}

This has no effect in the editor as there is no elelment that can contain all of these classes - they are split onto two levels of div.

Do we need the wrapper with .wp-block and the data-align? Blocks with no alignment don't have a wrapper, so I'm assuming it isn't used as a hook for the editor UI.

What is your proposed solution?

Remove the <div class="wp-block" data-align="full"> div and add the wp-block and alignment classes to the block level element.

E.g. <div class="wp-block wp-block-group has-background alignfull ..." ...>...</div>

If needed, the data-align attribute can be added in addition to the alignment class.

E.g. <div class="wp-block wp-block-group has-background alignfull ..." data-align="full" ...>...</div>