Privacy is necessary for changing, primping, and sexual relations.
There will be no separation between the two rooms, so Blanche will be a part of Stella and Stanley's sexual relationship, in a way, while she is present. Blanche feels uncomfortable with the arrangement, but has little choice in changing the situation. The photograph of Stanley as a military officer turns Stella on sexually. She voices her physical desire for him. This physical passion is one of their strongest ties in the relationship.