Most of the soiling in a carpet is loose, thats why vacuuming with a good vacuum cleaner is stressed as important. Get it out of the way.
Why?
What happens when you wet dirt?
It turns into a sludge.
Why do you want to be sucking sludge, especially if you dont have a high powered extraction machine.
When you got rid of the loose soil - and the Flexi5 and more expensive Host, or TM4 machines, have counter rotating brushes (CRB) which dig deep to get at the soiling and at the same time vac it up, then you can prespray. Use an Envirodri or the lowest priced Host or the Sebo Duo and you need to vac with say a Sebo BS36 etc.
Then you suck.
Purpose then of prespray is to loosen the stickey soil wrapped onto the fibres. Many presprays benefit from agitation, so you either brush it (get the heart pumping) of use an Enirodri, flexi5, Host, TM4, Oreck Orbiter, Sebo Duo, or your rotary with a brush or pad to do the agitation - get the prespray into the fibres so it can its job.