It's so you don't have to point nonstop on a Block, never letting go of the mouse button, to finish it off. You can pause, walk away, mine another Block ... whatever you want.
As of 1.3.2 this mod is completely networked. It works for multiple people on a server, you can take turns beating Blocks, and other peoples half-finished cracks will show for you.
Block Damage is limited to 30 Blocks at a time across the server. It is not saved, it does not interfere with your savegames. Damaged Blocks regenerate "slowly" after an initial short duration. [configurable]