It improves responsiveness and availability in busy production environments, where making a table unavailable for minutes or hours whenever you modify its indexes or column definitions is not practical.

It lets you adjust the balance between performance and concurrency during the DDL operation, by choosing whether to block access to the table entirely (LOCK=EXCLUSIVE clause), allow queries but not DML (LOCK=SHARED clause), or allow full query and DML access to the table (LOCK=NONE clause). When you omit the LOCK clause or specify LOCK=DEFAULT, MySQL allows as much concurrency as possible depending on the type of operation.

By doing the changes in-place where possible, rather than creating a new copy of the table, it avoids temporary increases in disk space usage and the I/O overhead of copying the table and reconstructing all the secondary indexes.