Is it recommended to have tab index navigation for complex web application?

I am doing requirement gathering for a complex in house web application. My customer is adamant to have keyboard tab index interaction for the application along with regular mouse interaction. My research says, there is no user who is physically challenged, may require tab index interactions.

What is the value add if we incorporate tab index interaction across application? What are the pro and cons to have tab index in a complex web application?

Thanks-