The majority of blade steel comes annealed which means the steel is in a state soft enough to drill, grind, sand, etc.. If you do not harden and temper it edge holding will be all but non existent. The way the elements that make up the steel are arranged on the atomic level give different forms of steel such as pearlite, martensite, austenite etc.. When done properly, heat treating arranges the elements in the form that gives the qualities desired for a given steel and its intended use. In a knife these qualities are usually hardness, toughness, and abrasion resistance. Hope this makes sense.