Dressing up for work is value signalling. "oh, a blazer? They are important." When it comes to client interactions, depending on the culture of the industry, being more professionally dressed is important. First impressions and all that. Self-confidence can be boosted by your appearance and that can help when interacting with folks. I've seen it work for folks that talk on the phone to customers. The other thing that helps boost confidence? Actually knowing what you are talking about :) I may not trust an investment representative as much if they are in their sweat pants compared to a suit. But for knowledge workers, I think there is a boon to being comfortable.