I think that the idea is that they scoop out the debris first and then flush to remove any remaining debris and to ensure that the downspouts are clear.

The unfortunate part is that if you read many of the on-line reviews of gutter cleaning services they are rife with complaints that they don't always perform the services that they are paid for. And given that the homeowner does not want to go up on a ladder to check, there is no easy way to tell unless you have overflow problems down the road.

I have some similar concerns about the guys that I have been using and this is borne out in some of their reviews but this seems to be a universal complaint.