A substantial number of passengers do connect into Barcelona for those reasons mentioned but also remember that both Reus and Girona are resorts in their own right during the summer and the latter is also a great access point to the Pyrenees and Andorra.
Their BCN operation initially seems very domestic based therefore assuming that they want to gain more of the Spanish domestic market compared to Girona which has a majority international set up.
I wouldn't bet against Ryanair expanding more into Europe though from it's BCN base.
One destination not mentioned in the launch and that stands out like a sore thumb is STN which I am sure will be one of the second wave expansion announcements along with DUB and HHN.