This is a question many of us ask from time to time. The simple answer is, like I mentioned above, that you can have fun and a good trip without a detailed plan, but on the long run if you want to make the most of it, you will need something to guide you and make you more focused and organised.
When we first start to practice teaching, we have to show our lesson plans to our university or mentor teachers, but when we start working on our own, sooner or later we just let them go. After teaching for several years, we mostly feel we know what and how we want to do in our lessons as we have taught that particular unit many times, and gradually we just leave the very detailed planning behind. Some of us might have their own notes in their Teacher’s Books, or little (sticky) notes in their course books to guide them through the different tasks. We have probably filed hundreds of photocopied materials to go with our teaching (and then never use them again because they’re hard to find or sort out conveniently). Sometimes we just have too many lessons a day to plan anything in greater details and go with “doorknob” planning and improvising along the book’s tasks. It’s all fine and can work well, but if you want to improve or just refresh your way of teaching (because you’re bored with teaching the same things again and again and it feels too much routine), turning to new perspectives with some detailed planning can help. Also, our groups are different and it can happen that a task that worked really well with one group won’t work with another one, and vice versa.
Another thing I keep saying to my teacher trainees is that we have to work before the lesson with preparing it in every imaginable detail, so in the lesson we needn’t work – there it is the students’ task to work. We are only there to guide them and help them when necessary. As with the example of going on a trip: after planning it well, we know the points and sights we want to show to our students, we anticipate the difficulties they might face (e.g. crossing a small stream, climbing up some rocks, ladders, etc.) and calculate how we might be able to help them, but we cannot walk instead of them. It’s their task to go on the journey and get over these difficulties. My final reason for planning a lesson is that anything can happen to you and in that case someone might have to substitute you. It’s also a great help for them if they have a kind of insight into how you work with your students and what you wanted to do in that lesson. So, the best lesson plans are the kinds from which anyone could teach, and if you bear it in mind when you write your lesson plan, and formulate your ideas as if you were giving instructions to someone who would teach this lesson instead of you, your plan will be detailed enough and structured, written in a language that is understandable for others.