For my agency (https://www.brotskydesigns.com), I found that client logos really help sell us. Customers tend to care about who your company serves more than more important things like quality or value.

For example, our first "big name" was Howie Mandel. We did his website. The website was pretty standard and no different, technically speaking, than any other client we had at the time but since it was a celebrity that most people have heard of we started getting a lot more clients and bigger companies or celebrities started taking us more seriously.