I have never used the 360 paper, but I suppose it’s the same technique used on vellum and other translucent papers.
The purpose is to get more variety of shades out of the same marker. For car renderings, they often render on the back side of the vellum to achieve a lighter, more subdued tone. It’s often used for car interiors where the objects are supposed to be behind the glass. Sometimes, they apply marker on both sides to get a even darker tone. You can do the same with chalk.
Most marker paper can’t be used on the flip side because it’s lines with water proof plastic sheet to prevent the ink from seeping through the paper. Eg will be letraset paper by tria.