I personally can't imagine not planning things out. A truly wonderful story, in my opinion, consists of small hints at what's to come in the future mixed with proper foreshadowing and, at times, references to characters who're yet to pop in to the story. Everything needs to flow well and naturally as you work towards your ultimate goals, and if the only "ultimate goal" you have is the ending, then most of the story, though it may be interesting, won't feel as built up to.
I don't plan every detail of every chapter, for that's rather redundant; it's fun and exciting to see just how things end up the way they are in a story. However, I plan out every major character and when they enter, their back stories, each plot twist I intend to have, major focal points of the fic, when and where my characters will catch specific Pokemon, when they'll fight their rivals and how the outcome will be, how that affects them, etc.
Basically, I plan out each of my characters, plan the plot and how it will affect them, when the plot will occur and when it will affect them, how far the journey should be after a set amount of chapters, how the ending will turn out, and what characters will die and when they will. Then, chapter by chapter, I just sort of... Fill in the blanks between all of those major moments however I please. Fun ideas come in and I'm able to work with those, but it never detracts from the over-all experience and I'm able to weave in foreshadowing and new characters without making it feel too sudden or awkward. That's the style that works for me, at least. If not planning anything ahead except one or two things works for other people, it's their stories and they're more than welcome to. I wish them the best and would love to see how well that works for them.