Wiring is a big Bug-a-Boo for a lot of people. Do you try to make it organized and neat from the start (preferred, but it slows down getting started) or do you try to neaten it up after running leads to things? (and then find out that your lengths, routing and/or turn radii suck). And what do you do later on when you inevitably add or change things - Neatly include it in the harness or tie-wrap it alongside, just to "get r dun"? Besides - Nobody's gonna see it, right?
And then there's that bit about; "Just wire it up. You don't need a wiring diagram. You'll remember where everything went." and we all know how THAT goes later on. And that includes noting the wire color - They offer different colored wires for a reason! Just ask @Alan Merklin (and others) how much fun it is figuring out wiring that someone else did when they only used red colored wires for the entire car!
In the industry, three people design harnesses: The product designer doesn't give a hoot about wire colors - All he/she cares about is signals, power and grounds from assembly A to B. He/she will never have to troubleshoot or fix it.
The manufacturing or field technician is the one who cares about colors because they need to find a fault fast. They intuitively want to know that grounds are always green (brown if you're German), power is always red (sometimes black, if you're English) and can trace a signal by wire color (usually by different stripes). The cable/harness designer is caught in the middle and is usually the one who chooses length, gauge, bends, anchor points and wire colors with input from the techs. While there are industry standards for all this, it seems like every company uses a lot of leeway once you get away from basic power and grounds and as long as they're consistent over time, that's OK.
Sorry - This got a little long. I used to live this stuff but even that doesn't guarantee that I always do a good job wiring things. Sometimes OK is good enough. 