The real reason why you want ->> can be seen in the code example for rule #7: instead of mapping the result of partitioning, it (wrongly) reuses a (i.e., the or

The real reason why you want ->> can be seen in the code example for rule #7: instead of mapping the result of partitioning, it (wrongly) reuses a (i.e., the original list). If bugs result in irregularity, then making regularity explicit (by using ->> or other macros/higher-order functions) can be a good thing.