Mr. Raymond tells Scout "you haven't seen enough of the world yet. You haven't seen this town, but all you gotta do is step back inside the courthouse." because:

Mr. Raymond explains to Scout that the world isn't so cheerful and kind like she wants it to be. Inside the courtroom is a chance to realize this. The courtroom will give Scout the most accurate insight available to understand not only the real world, but the citizens that live in her town.