Planning is important firstly because it keeps us on the right path towards our goal, and secondly it helps us keep our site clean and simple and not succumb to what I call the "Frankensite" effect.
Imagine building a house simply by finding random pieces of nice house fittings, some floor coverings that caught our eye, bathroom and kitchen fittings, doors, and door handles. All of these items picked individually without any thought for the style, size, or appearance of the other items. Then, after we have all the parts we want, finding a block of land, and assembling all the pieces randomly without any plan of what kind of house we want.
I am sure you can imagine the crazy, monstrous house that would result. To do this with a house would be unthinkable for most people.
Many people, however, build their web sites, and even their business in this way. They collect a big list of extensions they want to use and then try to work out what kind...