When a transaction is committed, the redo log entry in the redo log buffer must be written to disk. The characteristics of the redo log writes are very different from those of the data file I/O. The differences include:
Combined, these factors suggest that transaction processing is optimized if a redo log is on a dedicated disk device. Of course, if your database is primarily read-only, redo log I/O is unlikely to be an issue, and the redo logs can be placed in (virtually) any convenient location.