Reason #1: Safety. I've seen countless homeowners up on a ladder, using a power washer, trying to blast off some mildew up under the eaves of the home. Now, I applaud the courage and valor. . . but. . . is that really safe? Let's walk through this together. What happens when you pull the trigger on a power washer wand? There can be some serious kickback from the water pressure. And you're standing on a ladder 20' or more in the air when you pull that trigger! Please, for the love of your family, don't ever do this. The reason the homeowner is up on a ladder doing this in the first place is because he's (or she) is trying to 'power' wash the house. A true pro, using the soft wash method, has no need for, or use for, ladders. Everything is done from the safety of good ol' terra firma.
Now if you've just got a little dirt on the siding down at eye-level, then knock yourself out and go for it. Of course, please use low pressure (like garden hose level pressure) and clean the siding. But if you've got mildew, severe buildup, and/or hard to reach areas and heights, it is likely time to call in the pros so you're not tempted to do something unsafe.