The Importance of Credibility
What do witnesses and experts have in common? They both need to be credible.
You'd never put a witness on the stand if you couldn't trust their story. Similarly, clients are going to be less apt to trust you if they don't believe that you're capable of fulfilling the promises made by your practice. Television shows and movies have long showcased characters who somehow managed to get jobs that they weren't qualified to do, doing things like working as Spanish teachers without being able to speak the language or landing high profile design jobs while still being in high school. While this makes for a compelling sitcom fodder it's not an advisable course of action in the real world.
You need to ensure that your clients have no doubt about your capabilities. Diplomas, awards and mentions in the paper are all great but what's even better is when you can be shown as working to establish yourself as an expert in your field. When you willingly put yourself and your knowledge on display in a way that's informative as opposed to obnoxious you help build your credibility.