I think that they loved Jesus so much and wanted other converts to have the same kind of love for Him that they did. They went back to encourage the new believers to stand firm and not be swayed or scared by the hardships that may come their way. They wanted to show them that He was worth anything that happened due to their belief in Him.
'In this world you will have trouble, but take heart, I have overcome the world.' John 16:33
I'm sure that the new believers in the cities they revisited were glad that Paul and Barnabas came back. I think it reaffirmed what they had originally been taught. If He wasn't everything Paul said He was, then why in the world would he have risked his life just to come by and encourage them? Doesn't that make you think about what new believers in our time see when they look at our lives? If we say that we trust God completely but then we fall apart at the seams when the stock market takes a tumble or the price of gas goes up what is that showing them? What is it saying about our trust in Him?
Not too much I'd venture to say. Sometimes actions speak louder than words. So make sure that your actions are following your words. We are the light of the world, through Him, and I know I want to be a good ambassador of the torch I've been entrusted with!