Mixing old with the new adds interest to any room, and is unexpected. In the featured image, there is an antique buffet against the wall paired with a modern couch in a very traditional living room. Another example is that an old radio could act as a buffet or a night stand. Or secondhand chairs could be painted with a high gloss paint for side chairs as another idea. The room will feel unique and personal because of the old mixed with the new.
The less matchy the better as well. You will want to vary your shapes and materials. In the bedroom picture, it is unexpected for a quilt to be on the bed, when the headboard is sleek and traditional against the modern horizontal stripes on the walls.
In the dining room, you can see wrought iron with varying colors of wood to bring in different types of texture.
Just a couple of tips, and ideas for decorating your space.