Agitation is a very important part of the cleaning process. If using a CRB machine such as Host, Sebo, Envirodri, TM4 etc. etc. the benefits go far betond the distribution of the pre-spray. So, the benefits of agiitation in general and a CRB in
particular are:
More even distribution of pre-spray.
Shearing of soil from the fibre/yarn
Speading up the cleaning process on all but the lightest soil levels.
Lifting and "opening" of the pile.
This last benefit in particular has other benefits. It allows better penetration of the rinse, so by aiding soil and rinse agent removal. By increasing the efficiency of the rinse, it permits a lower water/rinse usage, fewer passes and hence quicker drying, And the pile on the finished, dry carpet stands prouder and, depending on chemistry used, has a better "hand".
My advice to Harvey, and others, is go for the best CRB machine you can afford. The agitation is far superior to what you can achieve scrubbing away with your wand or even a pile brush. The Sebo is a great place to start and should perform admirably in most residential situations, and at around £200 is a throw away product. The superior, but more expensive alterhatives are definately the way to go, but capital outlay is considerably more.
Safe and happy cleaning

Ken