The prespray should do most of the work. However, after the first pass with the wand you will have rinsed out most of the prespray. Some heavily soiled areas and pathways require multiple passes. If you don't have an emulsifier running through your solution, those extra passes are being made with plain water.
There may also be areas that did not receive any prespray - under furniture that later gets moved for example. These areas are usually not dirty enough that you need to go back and hit them with prespray. The cleaning power of the emulsifier will help get them clean.
Depending on the type of carpet you are cleaning and how much furniture you move, you may benefit from emulsifier or find that an acid rinse is better.
There are even a few products that give the benefits of a rinse agent - less residues, soft hand, carpet accepts some protectors better - and an emulsifier in the same product. That is what I like about End Zone.
Scott Warrington