Good cable management can be nice for a management and component servicing along with improved airflow, reduced places for dust buildup and improved airflow. Even where you think airflow wouldn't be affected with water cooling, it will be. If there's airflow in the case, you better believe that a rats nest of cables will collect dust from the air moving through them. And while it may only affect a handful of CFM of overall airflow...an improvement is an improvement regardless.
The other benefit I mentioned from good and smart cable management, component replacment can be much easier. Especially mainboard. The way I route cable management to keep mainboard access clean, this helps the onboard heatsinks with room to vent. When you have to find a place to stash a rats nest of wires when you're trying to stuff a mainboard in a case with a tighter fit, you'll be thankful for taking the time to manage the cables wisely. I don't spend more than an hour on cable management on my personal rigs...and half that for customer builds. I do a few basics, like zone bundling, so the cables in the same areas are zip tied and routed together, I place as much behind the MB tray as possible and push users to get 80+ modular PSU's
Maybe it's not something you need or understand, but in the region I live, dust is a common issue along with cheaper cooling solutions (not water cooling), where it's helpful to be easy to access and clean inside of a case and if a service is necessary that there's less time wasted with someone elses or your own mess of being lazy with the cable routing.
If cabe management was a waste, even the OEM's wouldn't do it...their internal case designs have shown for years that routing cables is helpful for cooling and service. Take a look inside a Dell, Lenovo, Gateway, etc....most, if not all built in the last 5-10 years have some sort of cable management solution adapted to their cases. I see no confusion as to why cable management is a solid long and short term set of solutions with a little bit of effort. Those that don't care, don't need to. But all they're doing is creating more work later on. Again this touches on smart cable management, because someone trying too hard to hide all the cables is also going to create more work depending on component to be replaced or if the PSU and cables need replaced. So being smart about it and thinking about how cables should be routed in order for service and "what-if's" and blending that with ensuring fan pathways aren't blocked, that the CPU and GPU area are CLEAR of any obstructions, that you have enough wiggle room in the wires to move them or tuck them out of the way for a component replacement. Just the simple things.
I believe in the utility of cable management first, which even if you focus on that will still provide good overall looks...it won't be perfectionist level, but it'll be very acceptable and not a headache to undo later on.
