As much as the prospect of putting your mattress on the floor might seem cheaper and easier, it is not something you should do. Placing your bed on the floor has a way of limiting the aesthetic feature of your room, it just does not bring out the best in it.
Also, when your bed is on some sort of frame, it helps to make it last longer. An elevation for your mattress helps the materials in it to breathe properly. This way, moisture would have no way of entering into the bed.