It also reminds you that the mission is bigger than life. There's so much stuff going on it's easy to forget what the real mission is. The slogans kind of keep it simple and remind you what it's all about when you're watching those boring Matt marriage scenes. With a show like this it would be easy to get lost with so many storylines. Unlike shows, like Lost, it's nice to be reminded every once in a while that things actually do fit in with the big picture and you're not wasting your time asking all the wrong questions.
Now that I've got into that, I think they've learned from Lost that's it's nice to focus somewhere and not just throw storylines together, asking more and more questions while doing a terrible job of simplifying things and getting answers. Too much information and too many things going on can frustrate viewers, especially those that can't keep up with every episode and every detail. As long as they still have a goal to focus on and put the show it all into perspective, a wider range of fans can enjoy it without feeling out of the loop. Knowing the details aren't as important as long as have a general idea of what's going on. So less-informed fans can still get in on the "Save the Cheerleader" mission or "Who's on the List" theme and enjoy the show for the just being a good show without having to feel like they have to had watched it from the beginning or understood every storyline and detail. I guess that makes it more fan-friendly. Sorry, my post sort of took on a life of it's own.