What special knowledge or skills about coffee do baristas have?
What you're asking is no different than: what special knowledge or certification/license do bakers have other than operating the oven? Many people know how to work the oven, but can they bake an artisan loaf? Do they even know the different kinds of bread? 

The same answer applies to baristas.  Some chains like *bucks have their own training program. Specialty shops chase baristas like the Neapolitan pizza parlors chase pizzaioli.  

The special skill? Besides the latte art which seems more of a North American thing, it is very difficult to pull a great espresso shot and choose good beans. I personally have been pulling shots on a pro machine daily for 7-years and still take my hat off when I meet a good barista. What you get from one of those Nesspresso or other superautomatics is the equivalent to microwave dinner lasagna vs Mario Batali's.