Why do layout planning?
It's easy to build a model railroad. Buy a book and pick a track plan. Lay some track and run trains. Easy as pie.
You may have built a layout using this technique. Maybe more than one. Or maybe you just put a sheet of plywood on sawhorses and started hooking track together. You had some sort of vision when you did this and the clarity of that vision affected the success of the layout. For it to satisfy for any length of time, it has to meet the needs that caused it to be built.
Thus the need for layout planning. The planning process will help to develop and refine your vision for your model railroad layout.
A good analogy to a model railroad layout is a vacation. Where do you want to go? How do you want to get there? What do you want to do along the way? What are you going to do when you get there? How long should you stay? Can you afford it? Who will watch the kids? Do you take them along? Will the car make it? Will you?
Some folks plan a vacation down to the Nth degree. Others grab a clean (optional) pair of jeans and a couple T-shirts and jump in the car.
So it goes with layout planning. Do enough planning that you are comfortable. You may have to make decisions along the way. You may have a detour or a breakdown en route. Or you may change your mind and deviate from your plan.
Or you might get lost. Getting lost is not necessarily bad. Some of the best times I've had come from getting lost. I call it a "scenic detour". Getting lost opens up new and unseen avenues.
You are doing this for fun, for enjoyment, because you want to. (If not, you are a professional and doing it for someone else. You better meet THEIR needs or you won't get paid.)
All sorts of wonderful things await you on this journey. The planning process is to give you an idea of the things that are out there. Things that may attract you. Things that you'd prefer to stay away from. (Tinplate and G-scale are that way to me, but not because I dislike them. They are an attractive nuisance and I have other things I'd rather do.)
Layout planning will cause you to explore a myriad of subjects. What railroad, era, locale, type of operation, do I want to model are all obvious first questions. But what about scale? Seems like a no-brainer. I have all this HO equipment. But I like running 30 and 40 car trains and the only space available is a 10 by 14 foot spare bedroom with closet and two windows that also has to serve as (pick one or more): computer room, sewing room, guest room, etc. Now N-scale becomes attractive or maybe a switching layout, shorter trains don't look so bad.
Space considerations. Unless you live alone, you'll have to negotiate for space. A land grab will likely cause that spare bedroom to gain another use, YOUR bedroom. But without all those pesky family obligations, think of all the time you'll have. Time. How long will it take to build this layout? How much time to maintain? Will I finish it in my lifetime? Do I want to and can I devote that much time to this? (Hello, spare bedroom.) Cost? All the same questions as for time. (Hello, poorhouse.) And can I even build what I design? Do I have the skills necessary? I don't know how to hand lay track, build benchwork, wire, sculpt rocks, etc., ad nauseum.
Daunted by all of this? You might decide to take an exploratory day trip or two to try out new techniques of acquire new skills. Get your feet wet by building a small layout from one of the "how to" books. Or maybe a diorama. Or a module such as N-TRAK. Experimentation such as this IS part of the planning process. One of these day trips could turn out to be the perfect vacation that you go on over and over again.
Anything you might possibly want to do can be done on a model railroad layout. Some combination of time, energy, money, technology, knowledge and ability will get it done. You have limited quantities of each of these available to you. By planning you make decisions about how to use these resources to maximize your enjoyment of the hobby.