Pre vaccing with a good quality, well maintained twin motor vacuum cleaner will not only remove dry particulate soil, but also open and lift the pile, making penetration of the pre-spray and extraction of the soil easier. It is accepted that the more dry soil removed, the less water and chemicals required. This has an effect on the quality of the job and also environmental consequences too.
I have a classic personal experience of the advantages of pre-vaccing. In a previous life
I didn't pre-vac
A 4 bedroom house I clean regularly would be an 08.30 start and a 16.00 finish. When I started to pre-vac, it took until 10.30 (including strategical placing of furniture) before I turned on the HWE. 2 hours lost? No way. This 4 bedroom house was now finished by 15.30 and the carpets dry much quicker. Why? because I use less water the carpets dry quicker. Because I use less water I don't spend as much time at the kitchen sink. Because I use less water, I dump in the loo less.
Water + dry soil = MUD
MUD is heavy and more difficult to remove.
I'll repeat what's been said by me and others in the past. Madam thought I'd already cleaned the carpet when all I'd done is pre-vacced it properly. Won't need much chemical/wet cleaning then will it
Safe and happy cleaning
Ken