In SQL Server, the functionality of non-clustered indexes is extended by adding non-key columns to the leaf level of the non-clustered index. Non-key columns can help to create cover indexes. By including non-key columns, you can create non-clustered indexes that cover more queries.

The Database Engine does not consider non-key columns when calculating the number of index key columns or index key size. Non-key columns can be included in the non-clustered index to avoid exceeding the current index size limitations of a maximum of 16 key columns and a maximum index key size of 900 bytes.

Another advantage is that using a non-key column in the index we can have index data types not allowed as index key columns generally.