It's used for temp tables, table variables, sort and hash spills (which you generally don't want because they're slower than hash/sort done in memory) and for work tables which the query processor may need.
There's not all that much monitoring of it possible in SQL 2000, the DMVs were only added in SQL 2005.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability