Indexes are important for query performance because a good set of indexes can provide more efficient access to tables and speed up query processing. Indexes can improve performance by providing local filtering, making join processing more efficient, avoiding the need for sort processing, providing index-only access, enforcing referential integrity, and more.
After the Workload Index Advisor generates recommendations for new indexes and changes to existing indexes, you can review those recommendations in detail. If you have the appropriate authorities on the database, you can run the DDL statements to carry out the recommendations that you select.