Planning usually makes most people's trips better, not worse.
It's not about figuring out the exact route, but making sure that you see what is there instead of driving by it because you weren't even aware something amazing was in that region.
It is often about saving money and getting more for your money. If you do your research, you can go to that nice family guesthouse that costs peanuts and has higher quality than the big and expensive hotels nearby.
And some really cool attractions actually require special passes/access/etc to set up before hand. If you just show up, you will be denied. So, for example, it would be rather stupid to show up on the wrong day and wait 2 weeks for the full moon because at the gate you learn that there is a moonlight tour (often with a moon lit rainbows). Or realize that you could've had a tour of some cool African diamond mine if you had arranged for permissions and tour a few weeks in advance. Things like that...
Particularly when you are on the road on a big trip, you have a lot of downtime to do a bit research. Sure, some people get paranoid and go overboard, but even then it's good: it relaxes people and they feel more confident when they set out on the road.
In summary, there are potentially very big downsides to minimal planning, and very minor upsides. Most people on the road have lots of downtime and it's not like they are "saving time" if they don't plan.