The BlueprintType specifier is the specifier that you will likely use most when working with UStructs. It exposes a struct to Blueprints and to general use within the Editor. For example, with this specifier, a struct can be added as a variable to a Blueprint.
The Blueprint graph context menu will also provide you options to make or break the struct.
You can also use the struct as a return value or parameter for BlueprintCallable and BlueprintPure functions.
Finally, you can read properties on the struct within a Blueprint graph, so long as those properties are marked BlueprintReadOnly. If the properties are marked BlueprintReadWrite, you can also edit those properties. (Conversely, you cannot mark a struct property as BlueprintReadOnly or BlueprintReadWrite if the struct is not BlueprintType.)