Currently (in vanilla), when a Wither spawns or takes damage, it will break blocks within and above itself, but not below itself.
With this config option enabled, not only will a Wither break blocks within and above itself, it will also break blocks below itself (including Obsidian used to trap the Wither).

This should cause the easy tactic of spawning the Wither trapped between Obsidian below and Bedrock above (like that found in the exit End Portal) to fail quickly, as the Wither will escape by breaking any Obsidian (or other blocks) found below itself when it takes damage.