Having once been on the customer's side needing my roof cleaned, I can say that I did it purely for aesthetics. It's really the only thing that the customer can immediately identify with as a real benefit. Think about it, how many customers really look at your work and go, "Wow thanks for stopping the granule loss" OR "Man I'm gonna possibly save an extra $10 bucks on my electricity now". Not likely. Clearly there are other justifications, some that you could make a good argument are even more important like extending roof life as you say, but you're not given the opportunity to make that case in a one line benefit statement.
That said, I think you might be able to craft a statement that covers aesthetics AND the damage it's doing to the roof.
Something like this
"THE #1 REASON TO CLEAN YOUR ROOF IS..." to wash away the ugly black algae stains causing damage to your shingles
Then maybe follow up with a sub-heading that says:
DON'T WAIT! Take Action Now! You'll be the envy of the neighborhood.
For many customers we're still going to have to do a lot of eduction in the pre-sale, not just why they should choose a roof cleaner that really knows what they're doing. With that in mind, we need to embrace it and not get frustrated with the ignorance, make them feel as though they are "one-upping" their neighbors with a service that others don't even really know about yet.