Since dirt tends to clings to paint, it does require some agitation regardless of whether or not you use a foam cannon. The problem with washing a panel is that it’s very easy to run out of suds before you finish. By using a foam cannon attached to a pressure washer lance, you can drastically reduce your wash time by both applying and rinsing off soap with the same device.
This is an even bigger deal if you are a mobile detailer that uses a pressure washer and water tank. Easier lubrication for removing dirt. Creates a slick surface to easily wash dirt away. Reduced risk of scratching paint. Avoid cross-contamination. Increase productivity. Use less detergent. No heavy buckets of water.