Have you ever walked into something knowing what outcome you wanted, but not knowing how to get there and thinking you'll figure it out along the way? Chances are that situation didn't end up going the way you wanted.
Now it is true that "failure" can come in varying degrees and is even a necessary step to success - but when you have a specific goal in mind, it can be disheartening to not achieve it.
It is also true that not even the best laid plans will guarantee success, but they certainly increase your chances. Not only will knowing what you want to do help guide you each step of the way, but it will aid in your preparedness for any snag or speed bump you may encounter.
When it comes to goal setting in the fitness world, the SMART principle is one that I've come across many times. SMART stands for Specific, Measurable, Attainable, Relevant, and Time-based. By setting SMART goals you will have your building blocks to creating plan for success because you will know what success looks like, you'll know how to measure it, and you'll know how long it should take you.
In short, you aren't be guaranteed success whether you have a plan or not. But having a well thought out plan will increase your chances attaining your goals.