Sun JoeSPX-FC34 34-Ounce Snow Foam Cannon for SPX Series Electric Pressure Washers, w/Transfer Adapter

The sun joe spx 3001 has a detergent chamber and mixer on the machine, why would i need this?

If you own a spx-3000 or the spx-3001. what purpose do this gun serve if you mix the soap in one of the cotainers on the pressure washer?

Bought a sun joe 1000 for my travel trailer. can you shut off the soap for a rinse-only spray? or do you have to remove it from the sprayer handle?

Will this foam cannon snap on right away to the sunjoe pressure washer without buying additional fittings ?