Dress down days are one of the daftest ideas to come out of the personnel management (human resources nowadays, of course) departments in a long time.
My staff wear whatever they want to wear when working. My receptionist is always "smartly dressed" because she chooses to dress like that and feels it is expected of her but there is no compulsion on my part for her to do that.
I don't employ very many people, but they can (within reason) wear whatever they like to work. They're not meeting clients,and there are no conventions to acknowledge.
As for 'dress-down' days - what a ludicrous idea. If a company wants people to donate to charity it could just ask, or even better, leave it up to indivduals to decide. It's not necessary to bribe staff to do it, as if they were children.
Is that it separates ones working life from ones private life.
This is actually quite useful.
Dress down days have some serious side effects. In a little bit of research I read a couple of years back, Employees were found to feel that choosing the correct outfit for "Dress Down" was extremely stressful and that many felt that there was an element of "Oneupmanship" involved.