It teaches the language. It has been around for several years now. It is pretty use full for some people to get the syntax of R down. Some online classes even have it report to them for extra credit etc.
The more I use R the less I use of base and mostly use the Hadley Wickham Universe of dplyr, tidyr, ggplot/ggvis etc.. This teaches mostly base and not the functional programming parts (Boy do I HATE loops and R base has a great list system that people seem to ignore)