Because internal promotions rarely happen as the result of an interview - despite what you imagine.
In a world that is increasingly focused on appearance and personal branding, your image counts just as much as your impact.
Trivial maybe, but true for many.
Women and men.
Dressing professionally, and well, indicates that you take yourself and your career seriously.
It demonstrates respect for each of your work environment, your craft or expertise, the people you work with and yourself.
It shows that you are professional and that you value your own contribution.