Helps to provide atomicity and durability guarantees (see CAP Theorem) in distributed systems.
A write ahead log is an append-only structure used for crash and transaction recovery. The WAL notably is a separate, additional data structure from the main storage and usually is disk-resident (i.e. stored on some form of persistent storage) rather than memory.
Compaction
After a certain amount of operations, the program using the WAL should perform a checkpoint, writing all the changes specified in the WAL to the database and clearing the log.