When trying to figure out how much work is being done on behalf of a query, we often want to take into account the full transactional impact.
SQL Server's parent-child model for transactions makes this extremely difficult, since child system transactions (e.g. worktables) are not tied to the parent transactions in any obvious way in the DMVs.
We need an easy way to get all of the information, in order to assist with performance tuning efforts.
(Note: There is a way to get this information, via sys.dm_tran_locks, but that DMV is so slow that the information is useless in any real-world situation.)