In simple language.
Pull up and pull Down resistors are needed to drop down the floating condition because sometimes when switches are connected to devices they may give floating state to device when they are in open loop. But devices sometime says i only accept one or zero so please pass me with those two things only so to lower down that problems we use pull up and pull down.wysiwyg_imageupload:12192:wysiwyg_imageupload:12193: