I was looking around cp_5gorge for fun. It occurred to me how I've never really fully appreciated in and soaked in TF2's maps, so I decided to load up a random map, turn off the HUD and viewmodel, and just walk around.
And then I realised just how much Valve loves shoving random nonsense pipes all over the walls of some of their maps. I think these things are all over certain places in HL2 as well. They come out one surface and go into another. No purpose, they make no sense, yet you usually just... don't notice them. Except when they're pointed out, then you realise they're everywhere.
Even the top iconic 2Fort spawnroom is home to a bunch of meaningless pipes.

Of course, the comfy rustic wooden parts of RED's fort is free from this phenomenon. But even within there, the moment you're out of the wooden areas you see these damn things.

The concrete BLU fort has pipes even in its main hallways.

In some places they make even less sense than usual.


This isn't even always consistent. Some maps have this in almost every room, others are nigh-devoid of these mystery pipes. Why? How did this practice start?
I understand conceptually why they do it - it adds some gameplay-unobtrusive visual clutter to a room that helps it feel less empty and more interesting. But it's strange to think about the level designer in Hammer, placing each of the prop entities, looking for the right pipe models to fit the pipes around the geo. The nature of Hammer means this can't have been a fast process. There'd have had to be some level of thought behind each one, some level of explicit intent. And that's fascinating to me.