Microsoft SQL Server 2012 Internals

Managing B-tree index structures

SQL Server maintains your indexes automatically, ensuring that the correct rows are there. As you add new rows, it automatically inserts them into the correct position in a table with a clustered index and adds new leaf-level rows to your nonclustered indexes that point to the new data rows. When you remove rows, SQL Server automatically deletes the corresponding leaf-level rows from your nonclustered indexes. Rows added to or removed from heaps also require that any nonclustered index indexes be adjusted.

So, although your indexes continue to contain all the correct index rows in the B-tree to help SQL Server find the rows you are looking for, you might still occasionally need to perform maintenance operations ...