Import and export your block settings if you want to reuse them across websites. For example, a large organization with multiple domain names will likely want a similar look and feel to their websites. To save time, you would export block settings from one and import them into the others. Then each separate site could be customized from there.
See also Reusing Blocks, Layouts, and Designs.
For details about block-specific options, see the list of block types in Blocks and Block Types or the specific articles linked from there.