Custom HTML Block

The Custom HTML block is for advanced users needing to add features and functionality beyond what the Gutenberg Block layout can accommodate. When its rendered on the page it has no containers or wrapping divs beyond what’s specified in the users markup.

One of the advantages is that you can replicate Guttenberg functionality such as Align Full/Wide by utilizing those classes. So if you wanted to have the contents of the block span to the width of the page you could add a “.alignwide” class.
Something to keep in mind. Since the block expects HTML and will try and rendering it as faithfullly as possible, WordPress will not filter the content and add paragraph tags or remove redundant space.