Steam Cleaners are not vacuum cleaners. You will need to vacuum your carpet as usual, but where carpet cleaners come in is to actually wash the carpet fibres in your carpet. This can give your carpets a new look and feel, though that obviously depends on the state of your carpets in the first place.
Some Steam Cleaners may not actually use steam to clean the carpets. What seems to be a common mechanism is that hot water and detergent are applied to the carpet, and then brushes might be involved in agitating the carpets to clean the fibres. Hot air is then often forced down into the carpet to help efficient cleaning. The dirty water is sucked back up into the carpet cleaner, and hot air used again to dry the carpets.
When you use a carpet cleaner, be aware that your carpet may require a few hours to dry before you can walk on it. However the results speak for themselves, with dirty carpets coming up as new.