It's a matter of maintaing enough flow for the water to carry the minerals off with it, as opposed to "misting" or soaker hoses.
Most of the water evaporates on the flat roof, but some runs down the gutters....In fact, the building owner will call us (A CPA who offices there) if he doesn't see water coming out of the gutter during hot weather.
Conversely, We service a retirement home with 200+ units....we replaced about 30 of them last year under a capital expenditure, because the facility irrigation system sprays a fine mist which DOES eat the coils out over time.
I used to listen to Mark Davis on WBAP in the mornings, and would cringe when he would advertise those misting systems for home AC units. I even sent him an email about it, which of course, he never answered.