Well see... an outline is like a road map.
Let's say you're taking a road trip, and you just get in your car and start driving. You don't know where you're going, so you just figure you'll drive around and see where you get. Or else perhaps you know where you want to get, but not how to get there, and so you just start randomly driving and hope you'll eventually get there.
Yeah, you might get there EVENTUALLY, but you're going to waste a lot of time (and gas

).
On the other hand, let's say you are going on a road trip, and you have the whole thing completely mapped and scheduled out. You know exactly what exits to take for the entire trip, you know just where you are going to take your bathroom breaks, you know exactly when you will arrive at each place.
Now, this is very good, yes? Well... what if you are driving along, and you see a shortcut? Or, let's say you see a beautiful scenic route that you really want to go on? Or an interesting state part you want to visit?
But NO! You must stick to the PLAN!
Well that's gonna be a boring trip.

How about this, instead: You plan out your route, leaving yourself some room detours and explorations. Then you start driving, keeping to the plan, but when you see something interesting or that looks better, you can take a detour and check it out. Explore it.
The key, is that when you are done with that exploration, you have a plan to return to. Or, if this detour changes your plan so drastically you can't return to it... well, no problem. Make a new plan. Maybe you even have a new destination. So you stop and plan how to get there, and start driving again.
That's how I see it.

Sure, some will tend more towards sticking to the plan, and others will deviate more from the plan. But it is always helpful to have a plan there to return to.