When there is no condition, they are always included for the properties, platforms, and hosts they are scoped for.
They can also be used for creating messaging rule templates that will adjust for all listings automatically.
For example, you may have check-in instructions that are identical for all listings, except for a paragraph about the lock-box that is different from listing to listing.
Instead of creating a different check-in instructions rule for every listing, you could create a specific custom code for each listing to hold this different paragraph about the lock-box, and include these custom codes in your rule. In this case, the custom code would not be tied to a condition.