The ability to produce "crisp" looking cushions which don't look "plumped" without having to resort to hard foams. You need to be careful with patterns and detailing, but Sunbrella is easier to work with and produce such results. You can do that with other materials (including Ultrasuede -- but it takes more skill.

The downside, is that materials isn't quite as soft. The upside, beyond initial appearance, is that the material wears "like iron." (Red wine spills and suntan lotion excluded :^)))) )