For avoiding the possible messy situation when you find out big change seems to be inevitable after site is already built, simulation is important. Drawing an apartment layout and arranging the furniture positions share the same concept of web site design. Both have size constraints and both have many elements to accomodate. There are many different possible combinations for arranging the furniture or elements, but the outcome only has two: Ugly or pretty. Say, you are living in a student housing village for quite a while, try to imagine how messy it can be when you are asked to move out temporarily for a big structural renovation? This really happened to me in last December. Through such simulation process, a web designer should have a more concrete mental picture in his mind before coding. Any professional and serious web designer should start his work this way.