Label blocks are used to "jump" to a separate flow in a Blend, they are the equivalent of a "go to" statement in coding.
Note: labels do not work like functions. After completing the flow under a label, the Blend execution will not return to the calling flow. If you want a function-like execution of blocks, you should use "Call Data Blend" to call another Blend from a Blend, or you could work with a Snippet.
Example usage of Label blocks: