When wsrep_debug is enabled, Galera Cluster logs more verbose information to the error log.
For example, when DDL is executed on a particular node, the node logs some information like this:
However, when wsrep_OSU_method is set to TOI and the DDL command is replicated to the other nodes in the cluster, those other nodes do not log anything about the DDL to their error logs, even if wsrep_debug is enabled on those other nodes.
It would be useful to see some kind of information in the error logs about replicated DDL when wsrep_debug is ON. The main use case is when trying to track exactly what DDL is being replicated to other nodes.