When you see the picture, you ignore the frame. When the picture is taken out of the picture, you see the bigger picture. The National Gallery's exhibition of picture frames from the Venetian empire may make you think that the establishment gallery has gone postmodern, but it gives you a context that no other exhibition on Renaissance art has given recently.
When you look at the exhibition at first glance, you think someone needs to call the police, by as you look closer, you see the work that went into the frame, how the figures were carved and the guilding against the varnished wood. While the picture frame seems mundane, you realise that the top may look like a proscenium arch of a theatre. It frames the story a painter tells in pigments. Go to any theatre and you could see where it comes from.
When you take a closer look, you realise there are motifs and symbols from classical antiquity, yet the scrolling suggests the parchment the craftsmen sketched his work on. The female figures with exposed breasts juxtaposed against religious works shows the sacred and profane were a common juxtaposition even then, only more subtle. In fact, when in mahogany, or other dark wood, you can see how it suggests an African woman from some obscure tribe. The lighter coloured wood suggests a more European looking, less exotic person. That said, I suspect these would be used when darker colours predominate in a painting.