Dual Lance
I'll speak from how we use it for house washing and sidewalk cleaning as:
First off, a dual lance is designed to draw your chemical from a bucket (or Chem tank) when you are downstreaming. When you turn the grip down it lowers the pressure so that the chemical injector can draw the chemical from your bucket. This saves a little time in not having to change tips.
We never use the the dual lance for down streaming, we use it for edging out concrete areas like driveways, sidewalks. When we do this we do not have a chemical injector hooked up. So when we lower the pressure using the grip, we are able to "control" our pressure and lower it, which allows us to get closer to the surface and clean, with out blowing mud, mulch, rocks, etc all over the place.
We also do this for brick surfaces, stone, block, and others, that are badly stained and have not been cleaned in a long time. With a higher volume (gpm) machine you can get closer to the brick and "flush" the stains out as opposed to using "pressure" to clean and risk blowing the mortar joints out or damaging the brick It's just another great way to Soft Wash brick and other hard porous surfaces. (note: I never recommend using any pressure on Stucco or EIFS,) A Dual Lance is also great for removing graffiti as you can control the amount of pressure you are using.
You can also achieve the above without a dual lance and use a ball valve, and turn the ball valve down, but you really need experience doing it this way so you don't choke your machine and cause damage to it.
We also use them for removing gum from concrete as, once, again, we can control the pressure right from the gun and being able to control the pressure almost immediately, helps in preventing damage.
They are also great for removing dirt daubers from brick as again, you can cut the pressure down, and get right up to the surface and "flush" all the stain out.
Toni, I know your a visual person, so here are a coupple pics. One is showing one of the guy's that attended my school edging out a driveway,(notice he stands in the grass to do it) couple more of me using the dual lance around window sills and inside the patio are that a lot of dirt daubers. Another is of me showing another student how to clean brick. In that one we are using a low pressure tip and using the ball valve to control pressure.
There lot's of great uses for these dual lances, once you start using one you'll love it.
Hope this helps.