Any professional carpet cleaner will tell you that a good pre-vacuum is a MUST before steam cleaning any carpets.
Why a pre-vacuum if your about to bring in this big powerful truck mount system that has tons of vacuum power?
The reason is simple. The more loose soil you remove before the pre-treatment and steam clean process the better clean you will have when you do your Steam Clean, because your vacuum and hot water can focus more on the nonloose dirt and your pre-spray can have time to work on loosening the dirt and particles that are not easily removed.
At the end of the day, it gives you more final cleaning power for better steam cleaning and a way better clean!