I notice today's blog post for attorneys from Cordell Parvin speaks to why credibility matters. He does a nice job of concisely spelling out what it takes to be perceived as credible. Check out the link below to read it for yourself.
http://www.cordellblog.com/client-development/what-is-credibility-and-why-does-it-matter/
Now I will take a stab at answering
Why does credibility matter?