Late answer but may be of use to other readers
DBCC DROPCLEANBUFFERS is an often used command for query testing and gauging query execution speed. This command (when run) leaves behind only the dirty pages, which is actually a small portion of data. It removes all the clean pages for an entire server.
Be advised that this command should not be run on production environment. Running this command will result in mostly empty buffer cache. Running any query after executing the DBCC DROPCLEANBUFFERS command, will use physical reads to bring back the data into the cache, which is very likely going to be a lot slower than memory.
Again, treat this command similarly to DBCC FREEPROCCACHE - it should not be run on any production server unless you absolutely know what you are doing.
This can be a useful development tool because you can run a query in a performance testing environment over and over without any changes in speed/efficiency due to caching of data in memory.
Learn more at: http://www.sqlshack.com/insight-into-the-sql-server-buffer-cache/