I am, without shame or reservation, an outliner. I tried, some time ago, to be an improviser. I really went for the whole, "Start writing and see where it goes." thing. For me, where that goes is a nosedive into the ground somewhere around the 20,000 word mark.
So I learned how to outline. I embraced outlining, like a religion.
And it made a huge difference.
One big reason was, I was getting hung up on the mysteries. On my failed attempt to write an ongoing USS Banshee story, for example, I had a whole thing where my main character, Lt. Samantha Kengle, had been praised and honored for her actions in a battle a few months prior. And I hinted all this stuff about how what everyone THINKS happened and what she REALLY did were two very things.
1. I had NO CLUE what everyone thinks happened.
2. I had NO CLUE what she really did.
Thus, I had to go back to the drawing board. And plan.
One thing I haven't had a problem with is the notion that outlining stifles the writing process. In every finished (or near finished) work, I've strayed from the outline in minor ways. Or even major ways. I've made plenty of discoveries about the characters, and how to get what I need to happen.
The best analogy, for me, is making a cross-country drive. The outline is looking at the map, planning what route I'm going to take to get where I need to go. The writing is the actual drive, where actual discoveries along the way (Route 85 is shut down? World's Biggest Pecan Pie?) cause deviation... but since I have the plan... I know how to make it to the end.