why do we use index in sql code example

Example: sql indexes

CREATE INDEX
Creates an index named ‘idx_test’ on the first_name and surname columns of
the users table. In this instance, duplicate values are allowed.
CREATE INDEX idx_test
ON users (first_name, surname);
CREATE UNIQUE INDEX
Creates an index named ‘idx_test’ on the first_name and surname columns of
the users table. In this instance, duplicate values are allowed.
CREATE UNIQUE INDEX idx_test
ON users (first_name, surname);
DROP INDEX
Creates an index named ‘idx_test’ on the first_name and surname columns of
the users table. In this instance, duplicate values are allowed.
ALTER TABLE users
DROP INDEX idx_test;