What problem does this address?

At "tablet" sizes, the columns block snaps to a 50/50 view:

(From the Stewart theme: Automattic/themes#4369)

When columns are used for structural elements of the page this is not always what we need. It is possible to disable this by turning off "stack on mobile", but that setting is still useful in these scenarios.

What is your proposed solution?

It would be good to have a block attribute which would disable this behaviour on the block. It needn't be surfaced in the UI.