What is the significance of Pennytree being a royal fief in the Riverlands? Also, why didn’t the Targaryens ever have any lands outside the Crownlands?

I think we’ll have to wait for GRRM’s “Village Hero” story to find out, but odds are that Egg had Pennytree made a royal fief to honor his mentor’s former master. 

And Summerhall is in the Stormlands.