Never played Thief, but I logged a lot of hours in Unreal, 1998. I was (and still am) amazed at how full-featured the software renderer was. I always wished I could peek at the code behind it.
If memory serves, the only thing my 3dfx Voodoo3 could do that software-only mode could not was surface reflections. Maybe something with colored lighting too, it's been a long time. Point is, it was a decent enough substitute for dedicated graphics hardware.
There are definitely outside organizations whose sole responsibility is to monitor TV/radio broadcasts and nowadays even podcast sponsor breaks to ensure that ad copy is inserted as agreed. (Source: I was contracted to do some of that work for a time.)
I'm the person who reimplemented Cosmo's Cosmic Adventure (DOS, 1992) and my original reasoning was a desire to know how it was able to do some of the graphical tricks it did on such underpowered hardware (it could run on an IBM AT). The game wasn't anything special by any metric, but it was an important piece of my childhood and I felt an attachment to it. I also learned a hell of a lot about the PC platform, the C ecosystem from the 80s, and my own tastes as an engineer.
I started by reading disassembled real-mode x86 code. Not bad, not great. The thing that took me the longest to “get” was how much was implicitly coded into each instruction. Once I figured out how the accumulator worked, plus the way the flags influenced jump instructions, I found over time that most other architectures are more similar than they are different.