Even if you buy your lumber already planed, you will still encounter many circumstances that require the use of a planer. For example, you might want to build a simple and delicate jewelry box out of small scrap pieces lying around the shop, and you will end up making a small and clunky jewelry box because all of your lumber is 3/4″ thick, and that’s how it is going to stay. That is just the first example. Think about all of the other times that you will pick up a piece of lumber in the shop and it will be the wrong thickness, either just slightly wrong or in an entirely different size category. A planer is a real problem solver and can fix all of that.
Let’s be clear about one thing: No workshop is perfect for everyone. If there were, I could simply give you a precise plan to follow with clear dimensions. Every person who comes to the hobby, avocation, or profession of woodworking has his own particular collection of tools and his own peculiar work style, skills, and desires for the kind of woodworking he wants to do that gives him a unique approach as to how he will lay out a shop: where the tools should go, where lumber and supplies are stored, and how much bench area he needs.