Suppose we have 1000 records in a table and 400 records deleted now . but indexes are exists for that 400 records . So when we will search any records then Pointer will rotate on all 1000 indexes. So in that case we should use UPDATE_STATISTICS that will delete 400 indexes. same senirio will be follow in insert and update records. Basically UPDATE_STATISTICS use for refresh table indexes

This command is basically used when we do a large processing of data. If we do a large amount of deletions any modification or Bulk Copy into the tables, we need to basically update the indexes to take these changes into account. UPDATE_STATISTICS updates the indexes on these tables accordingly.