Federal clerkship value

Preface-I have an office practice. I couldn't find the Federal courthouse with a map.

My question is this. Litigators I have worked with hold lawyers who have served as clerks to Federal judges in high regard when hiring associates. Why is that? Is it just a presumption that they are smarter? Or is it the idea those clerks will carry more weight in federal court? Or that they gain special experience as clerks?

We are looking to hire a Federal clerk, and he wants a pretty hefty premium due to his clerkship. I'm trying to understand why.