Some monitoring tools turn this on.
http://logicalread.solarwinds.com/why-dpa-uses-sql-server-trace-flag-2861-and-zero-cost-plans-tl01/#.VF5CMvnF9Ps
From Microsoft Technet:
SQL 8 – Cache query plans for queries that have a cost of zero or near to zero.
When turned on, fn_get_sql function can return the SQL text for activities that have zero cost plans kb 325607
Tip: Avoid Using Trace Flag 2861 to Cache Zero-Cost Query Plans
http://social.technet.microsoft.com/wiki/contents/articles/13105.trace-flags-in-sql-server.aspx
http://www.microsoft.com/technet/abouttn/subscriptions/flash/tips/tips_042705.mspx
Basically if you have a very very busy server, you should turn this off. Otherwise, if it not making an appreciable difference, it can benefit some monitoring tools.