Futons and platform beds certainly have a lot in common, but they are two different types of bedding that were borne from different cultures. Both futons and platform beds use only a mattress or cushion rather than the more common mattress with box spring, and both types of furniture generally lie low to the ground. While both futons and platform beds are similar in style, the futon was designed by the ancient Japanese as a bed meant to be stored during the day. It is lightweight, compact, and it was traditionally placed on a bare floor. Platform beds are not designed to be stowed away, and they do not lie on a bare floor.
A platform bed is a mattress that lies directly on a hard, horizontal platform or base. The base provides support that a box spring would otherwise provide, and the platform is often low to the ground. Some platform beds have storage drawers built into the base, and the base itself can be made from wood or metal. One of the differences between futons and platform beds is the base; traditionally, futons did not have a base at all. They were simply a cushion and a comforter that was placed directly on the floor. The floor itself would act as a base. While modern futons do come with a frame of sorts, the most basic version of the futon did not historically come with one.
A futon was also traditionally a very thin cushion rather than a thick mattress. This was because the mattress was meant to be stowed in a closet so the bedroom could be used for other purposes during the daytime. Today, futons are generally thicker mattresses, and they come with a folding metal or wooden frame. The futon cushion is flexible enough to fold into a couch for use during the daytime. At night, the frame can be folded down into a flat position so the futon can be used as a mattress.
One of the most fundamental differences between futons and platform beds is functionality. A platform bed is specifically a bed; a modern futon is meant to be both a bed and a couch. The futon design has changed considerably over the years, but the idea of a storable or changeable piece of furniture has remained consistent. A platform bed has always been just a bed, though it differs considerably from a traditional mattress and box spring set up. The futon and platform bed resemble each other at first glance, but their purposes are quite different.